diff --git a/core/automation/schedule.py b/core/automation/schedule.py
new file mode 100644
index 00000000..e3b8e37b
--- /dev/null
+++ b/core/automation/schedule.py
@@ -0,0 +1,303 @@
+"""Pure functions for computing the next-run datetime of a scheduled
+automation trigger.
+
+The Auto-Sync schedule board currently exposes interval-based scheduling
+(``every N hours``) backed by ``trigger_type='schedule'``. The
+automation engine ALSO supports ``daily_time`` and ``weekly_time``
+triggers via separate ``_setup_*_trigger`` methods inline on the engine
+class. None of that logic is currently testable in isolation — the
+engine's ``_finish_run`` reaches for ``datetime.now()``, threads it
+through ``_next_weekly_occurrence``, and writes the result to the DB,
+all on the same call.
+
+This module lifts the "given a trigger config, what's the next run?"
+question out of the engine into a pure function:
+
+    next_run_at(trigger_type, trigger_config, now_utc, default_tz)
+        -> Optional[datetime]
+
+That means:
+- ``now_utc`` is INJECTED, not pulled from the system clock. Tests
+  freeze time without monkeypatching ``datetime.now``.
+- ``default_tz`` is INJECTED. Daily / weekly / monthly schedules are
+  inherently in the USER'S timezone (cron "every Monday at 9am" is
+  not UTC), and the historic engine implicitly used the server's
+  local tz via naive ``datetime.now()``. That broke for users on a
+  different tz than their server. The pure function takes the tz
+  explicitly so the caller controls it.
+- Returns an aware UTC ``datetime`` ready to serialise to the DB's
+  ``next_run`` string column, or ``None`` for unrecognised /
+  event-based triggers (engine should not store a next_run for those).
+
+PR 1 of the schedule-types feature ships ONLY this module + tests.
+The engine continues to compute next_run via its existing inline
+helpers; PR 2 collapses those into a single ``next_run_at`` call.
+Net behavior is identical until the engine is wired through — this
+PR is pure plumbing.
+
+Schedule types supported here:
+
+- ``schedule`` (interval): ``{interval: N, unit: 'minutes'|'hours'|'days'}``
+  — adds the interval to ``now_utc``; no tz needed.
+- ``daily_time``: ``{time: 'HH:MM', tz: '<IANA>'}`` — runs every day at
+  the given local time in the given timezone. ``tz`` falls back to
+  ``default_tz`` when absent.
+- ``weekly_time``: ``{time: 'HH:MM', days: ['mon','wed',...], tz: '<IANA>'}``
+  — runs on the matching weekday(s) at the given local time. Empty
+  ``days`` list means "every day" (matches the engine's existing
+  fallback in ``_next_weekly_occurrence``).
+- ``monthly_time``: ``{time: 'HH:MM', day_of_month: 1-31, tz: '<IANA>'}``
+  — runs on the given day each month. Days that don't exist in a
+  given month (Feb 30, Apr 31) clamp to the LAST valid day of that
+  month rather than skipping the run entirely; missing a whole
+  month silently because the schedule was over-eager is worse than
+  running a day early.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timedelta, timezone
+from typing import Any, Dict, Optional
+
+try:
+    from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
+except ImportError:  # pragma: no cover — zoneinfo ships with 3.9+
+    ZoneInfo = None
+    ZoneInfoNotFoundError = Exception
+
+
+# Weekday abbreviation → ``datetime.weekday()`` index (Mon=0..Sun=6).
+# Mirrors the engine's existing ``_next_weekly_occurrence`` mapping so
+# schedules created against either implementation accept the same
+# ``days`` strings.
+_WEEKDAY_MAP = {
+    'mon': 0, 'tue': 1, 'wed': 2, 'thu': 3, 'fri': 4, 'sat': 5, 'sun': 6,
+}
+
+_INTERVAL_MULTIPLIERS = {
+    'minutes': 60,
+    'hours':   60 * 60,
+    'days':    60 * 60 * 24,
+    'weeks':   60 * 60 * 24 * 7,
+}
+
+
+def next_run_at(
+    trigger_type: str,
+    trigger_config: Dict[str, Any],
+    now_utc: datetime,
+    default_tz: str = 'UTC',
+) -> Optional[datetime]:
+    """Compute the next-run timestamp (UTC, aware) for a scheduled
+    trigger. Returns ``None`` for unrecognised types or event-based
+    triggers — callers should not write a next_run for those.
+
+    See module docstring for supported trigger types + config shapes.
+    """
+    if not isinstance(trigger_config, dict):
+        trigger_config = {}
+
+    if trigger_type == 'schedule':
+        return _next_interval(trigger_config, now_utc)
+    if trigger_type == 'daily_time':
+        return _next_daily(trigger_config, now_utc, default_tz)
+    if trigger_type == 'weekly_time':
+        return _next_weekly(trigger_config, now_utc, default_tz)
+    if trigger_type == 'monthly_time':
+        return _next_monthly(trigger_config, now_utc, default_tz)
+    return None
+
+
+# ---------------------------------------------------------------------------
+# Interval
+# ---------------------------------------------------------------------------
+
+
+def _next_interval(config: Dict[str, Any], now_utc: datetime) -> datetime:
+    """``{interval: N, unit: 'hours'}`` → ``now_utc + N hours``.
+
+    Mirrors the engine's existing ``_calc_delay_seconds``. Unit defaults
+    to ``hours`` for backward compat with legacy DB rows that pre-date
+    the unit field being mandatory; interval defaults to 1 so a fully
+    empty config doesn't divide-by-zero or schedule for the past."""
+    try:
+        interval = max(int(config.get('interval', 1)), 1)
+    except (TypeError, ValueError):
+        interval = 1
+    unit = config.get('unit') or 'hours'
+    seconds = interval * _INTERVAL_MULTIPLIERS.get(unit, _INTERVAL_MULTIPLIERS['hours'])
+    return _ensure_utc(now_utc) + timedelta(seconds=seconds)
+
+
+# ---------------------------------------------------------------------------
+# Daily
+# ---------------------------------------------------------------------------
+
+
+def _next_daily(
+    config: Dict[str, Any], now_utc: datetime, default_tz: str,
+) -> datetime:
+    """``{time: 'HH:MM', tz: '<IANA>'}`` → next occurrence of that
+    wall-clock time in the user's timezone, expressed as aware UTC.
+
+    DST-aware via ``zoneinfo``: when the local time falls during a
+    spring-forward gap, the ``replace`` lands on a non-existent
+    instant; ``zoneinfo`` resolves that to the gap's later side
+    (e.g. 02:30 on the DST-forward day becomes 03:30 local). Tests
+    pin both spring-forward and fall-back behaviour."""
+    tz = _resolve_tz(config.get('tz') or default_tz)
+    hour, minute = _parse_hhmm(config.get('time'))
+    now_local = _ensure_utc(now_utc).astimezone(tz)
+    target_local = now_local.replace(hour=hour, minute=minute, second=0, microsecond=0)
+    if target_local <= now_local:
+        target_local = target_local + timedelta(days=1)
+    return target_local.astimezone(timezone.utc)
+
+
+# ---------------------------------------------------------------------------
+# Weekly
+# ---------------------------------------------------------------------------
+
+
+def _next_weekly(
+    config: Dict[str, Any], now_utc: datetime, default_tz: str,
+) -> datetime:
+    """``{time: 'HH:MM', days: ['mon',...], tz: '<IANA>'}`` → next
+    occurrence of that wall-clock time on any of the listed weekdays
+    in the user's timezone.
+
+    Empty ``days`` list ≡ every day, matching the engine's existing
+    fallback. Unrecognised day abbreviations are silently dropped
+    (an empty result-set then triggers the every-day fallback)."""
+    tz = _resolve_tz(config.get('tz') or default_tz)
+    hour, minute = _parse_hhmm(config.get('time'))
+    days = _parse_weekdays(config.get('days'))
+
+    now_local = _ensure_utc(now_utc).astimezone(tz)
+    # Scan today + next 7 days; the matching day with a future
+    # local time wins. 8-day scan is enough to handle the case where
+    # today already passed the time AND today is the only allowed
+    # weekday (next occurrence is exactly one week out).
+    for offset in range(8):
+        candidate = now_local + timedelta(days=offset)
+        if candidate.weekday() not in days:
+            continue
+        target = candidate.replace(hour=hour, minute=minute, second=0, microsecond=0)
+        if target > now_local:
+            return target.astimezone(timezone.utc)
+    # Shouldn't reach: 8-day scan always finds a hit when ``days``
+    # is non-empty. Defensive fallback: next week, same weekday as today.
+    fallback = (now_local + timedelta(days=7)).replace(
+        hour=hour, minute=minute, second=0, microsecond=0,
+    )
+    return fallback.astimezone(timezone.utc)
+
+
+# ---------------------------------------------------------------------------
+# Monthly
+# ---------------------------------------------------------------------------
+
+
+def _next_monthly(
+    config: Dict[str, Any], now_utc: datetime, default_tz: str,
+) -> datetime:
+    """``{time: 'HH:MM', day_of_month: 1-31, tz: '<IANA>'}`` → next
+    occurrence in the user's timezone.
+
+    ``day_of_month`` is clamped to ``[1, 31]``. When the target day
+    doesn't exist in a given month (Feb 30, Apr 31), the schedule
+    falls back to the LAST valid day of that month — running a day
+    or two early in short months is less surprising than skipping
+    a month entirely. This matches the convention every cron
+    implementation in the wild settled on."""
+    tz = _resolve_tz(config.get('tz') or default_tz)
+    hour, minute = _parse_hhmm(config.get('time'))
+    raw_day = config.get('day_of_month', 1)
+    try:
+        target_day = max(1, min(31, int(raw_day)))
+    except (TypeError, ValueError):
+        target_day = 1
+
+    now_local = _ensure_utc(now_utc).astimezone(tz)
+    # Try this month first; if the target day has already passed
+    # (or doesn't exist this month and the clamped day is in the
+    # past), advance to next month. Loop bounded to 12 iterations
+    # so a pathologically broken config can't infinite-loop us.
+    year, month = now_local.year, now_local.month
+    for _ in range(12):
+        day = min(target_day, _days_in_month(year, month))
+        target = now_local.replace(
+            year=year, month=month, day=day,
+            hour=hour, minute=minute, second=0, microsecond=0,
+        )
+        if target > now_local:
+            return target.astimezone(timezone.utc)
+        # Roll to next month.
+        if month == 12:
+            year, month = year + 1, 1
+        else:
+            month += 1
+    # Defensive — should be unreachable.
+    return (now_local + timedelta(days=30)).astimezone(timezone.utc)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _ensure_utc(dt: datetime) -> datetime:
+    """Coerce a possibly-naive datetime to aware UTC. Naive inputs
+    are assumed UTC (matches the convention the engine uses when
+    parsing the DB ``next_run`` column)."""
+    if dt.tzinfo is None:
+        return dt.replace(tzinfo=timezone.utc)
+    return dt.astimezone(timezone.utc)
+
+
+def _resolve_tz(name: Optional[str]):
+    """Look up an IANA tz by name. Falls back to UTC when zoneinfo
+    isn't available (3.8 / minimal images) or the name is unknown."""
+    if not name:
+        return timezone.utc
+    if ZoneInfo is None:
+        return timezone.utc
+    try:
+        return ZoneInfo(name)
+    except ZoneInfoNotFoundError:
+        return timezone.utc
+
+
+def _parse_hhmm(time_str: Optional[str]) -> tuple:
+    """Parse ``HH:MM`` → ``(hour, minute)``. Defaults to 00:00 on
+    garbage input — same defensive shape as the engine's existing
+    daily/weekly time parsing."""
+    if not isinstance(time_str, str):
+        return 0, 0
+    try:
+        h, m = time_str.split(':', 1)
+        return max(0, min(23, int(h))), max(0, min(59, int(m)))
+    except (ValueError, AttributeError):
+        return 0, 0
+
+
+def _parse_weekdays(days) -> set:
+    """``['mon', 'wed']`` → ``{0, 2}``. Empty / missing / all-invalid
+    list returns ``set(range(7))`` ("every day"), matching the
+    engine's existing ``_next_weekly_occurrence`` fallback."""
+    if not isinstance(days, (list, tuple)):
+        return set(range(7))
+    parsed = {_WEEKDAY_MAP[d.lower()] for d in days
+              if isinstance(d, str) and d.lower() in _WEEKDAY_MAP}
+    return parsed or set(range(7))
+
+
+def _days_in_month(year: int, month: int) -> int:
+    """Last calendar day of ``year-month``. Stdlib-only — no calendar
+    module import needed; cycle through the 12 months."""
+    if month == 12:
+        next_first = datetime(year + 1, 1, 1)
+    else:
+        next_first = datetime(year, month + 1, 1)
+    last_day = next_first - timedelta(days=1)
+    return last_day.day
diff --git a/requirements.txt b/requirements.txt
index 73ef28a7..4a94d257 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -28,6 +28,12 @@ beautifulsoup4==4.14.3
 # System monitoring
 psutil==7.2.2
 
+# IANA timezone data — required by ``zoneinfo`` on Windows hosts and
+# minimal Docker base images that ship without the system tz database.
+# Consumed by ``core/automation/schedule.py`` for daily / weekly /
+# monthly schedule next-run computation in the user's local timezone.
+tzdata==2026.2
+
 # YouTube support -- unpinned; yt-dlp must track upstream releases to stay functional
 yt-dlp>=2026.3.17
 
diff --git a/tests/automation/test_schedule.py b/tests/automation/test_schedule.py
new file mode 100644
index 00000000..a73782ff
--- /dev/null
+++ b/tests/automation/test_schedule.py
@@ -0,0 +1,383 @@
+"""Tests for ``core/automation/schedule.py:next_run_at``.
+
+Pure function over (trigger_type, trigger_config, now_utc, default_tz)
+so each case can pin a single rule without monkeypatching the system
+clock. Covers the existing engine behaviour (interval, daily, weekly)
+plus the new ``monthly_time`` shape PR 1 introduces.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timedelta, timezone
+from zoneinfo import ZoneInfo
+
+import pytest
+
+from core.automation.schedule import next_run_at
+
+
+# ---------------------------------------------------------------------------
+# Helper — clear, timezone-aware datetime construction in test bodies.
+# ---------------------------------------------------------------------------
+
+
+def _utc(year: int, month: int, day: int, hour: int = 0, minute: int = 0) -> datetime:
+    """Aware UTC datetime — every ``now_utc`` injection in tests
+    flows through this so a stray timezone bug is impossible."""
+    return datetime(year, month, day, hour, minute, tzinfo=timezone.utc)
+
+
+# ---------------------------------------------------------------------------
+# Dispatcher: trigger_type routing.
+# ---------------------------------------------------------------------------
+
+
+def test_returns_none_for_unrecognised_trigger_type():
+    """Event-based / unknown trigger types are not scheduled — the
+    caller should NOT write a next_run for them."""
+    now = _utc(2026, 5, 27, 12, 0)
+    assert next_run_at('event', {}, now) is None
+    assert next_run_at('garbage', {'interval': 1}, now) is None
+    assert next_run_at('', {}, now) is None
+
+
+def test_returns_none_for_non_dict_config():
+    """Defensive — callers may pass through whatever ``json.loads``
+    returned. Non-dict configs trigger the fallback path which is
+    'treat as empty dict + use defaults'."""
+    now = _utc(2026, 5, 27, 12, 0)
+    # Interval-typed with garbage config falls back to defaults
+    # (interval=1, unit='hours') rather than crashing.
+    result = next_run_at('schedule', None, now)
+    assert result == now + timedelta(hours=1)
+
+
+# ---------------------------------------------------------------------------
+# Interval (``trigger_type='schedule'``)
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize('unit,seconds_per_unit', [
+    ('minutes', 60),
+    ('hours',   3600),
+    ('days',    86400),
+    ('weeks',   86400 * 7),
+])
+def test_interval_units(unit, seconds_per_unit):
+    """Every supported unit scales the interval correctly."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('schedule', {'interval': 3, 'unit': unit}, now)
+    assert result == now + timedelta(seconds=3 * seconds_per_unit)
+
+
+def test_interval_unknown_unit_defaults_to_hours():
+    """Backward compat with DB rows whose ``unit`` field is missing
+    or an unrecognised value — engine's historic behaviour was to
+    treat as hours, and we preserve that."""
+    now = _utc(2026, 5, 27, 12, 0)
+    assert next_run_at('schedule', {'interval': 2, 'unit': 'fortnights'}, now) == now + timedelta(hours=2)
+    assert next_run_at('schedule', {'interval': 2}, now) == now + timedelta(hours=2)
+
+
+def test_interval_clamps_zero_and_negative_to_one():
+    """Without a floor a zero/negative interval would schedule for
+    the past or fire instantly in a loop. Engine clamped to >=1 via
+    ``max(int(interval), 1)``; we preserve that contract."""
+    now = _utc(2026, 5, 27, 12, 0)
+    assert next_run_at('schedule', {'interval': 0, 'unit': 'hours'}, now) == now + timedelta(hours=1)
+    assert next_run_at('schedule', {'interval': -5, 'unit': 'hours'}, now) == now + timedelta(hours=1)
+
+
+def test_interval_garbage_interval_falls_back_to_one():
+    """Non-numeric ``interval`` → default of 1. Survives a JSON column
+    where the field was typed as a string by an old admin script."""
+    now = _utc(2026, 5, 27, 12, 0)
+    assert next_run_at('schedule', {'interval': 'oops', 'unit': 'hours'}, now) == now + timedelta(hours=1)
+
+
+def test_interval_ignores_tz_field():
+    """Interval scheduling is wall-clock-independent — adding 6 hours
+    is the same in every timezone. The ``tz`` field is ignored even
+    if a caller mistakenly sets it."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('schedule',
+                         {'interval': 6, 'unit': 'hours', 'tz': 'America/Los_Angeles'},
+                         now)
+    assert result == now + timedelta(hours=6)
+
+
+# ---------------------------------------------------------------------------
+# Daily (``trigger_type='daily_time'``)
+# ---------------------------------------------------------------------------
+
+
+def test_daily_today_at_future_time_runs_today():
+    """It's 12:00 UTC and the schedule says 18:00 UTC — next run is
+    today at 18:00, not tomorrow."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('daily_time', {'time': '18:00', 'tz': 'UTC'}, now)
+    assert result == _utc(2026, 5, 27, 18, 0)
+
+
+def test_daily_today_at_past_time_runs_tomorrow():
+    """It's 18:00 UTC and the schedule says 09:00 UTC — next run is
+    tomorrow at 09:00."""
+    now = _utc(2026, 5, 27, 18, 0)
+    result = next_run_at('daily_time', {'time': '09:00', 'tz': 'UTC'}, now)
+    assert result == _utc(2026, 5, 28, 9, 0)
+
+
+def test_daily_at_exact_target_time_runs_tomorrow():
+    """Edge case: schedule fires at exactly 09:00, and ``now`` is
+    exactly 09:00. ``<=`` check pushes to tomorrow — otherwise we'd
+    immediately reschedule for the present moment and the engine
+    would run again in 0s."""
+    now = _utc(2026, 5, 27, 9, 0)
+    result = next_run_at('daily_time', {'time': '09:00', 'tz': 'UTC'}, now)
+    assert result == _utc(2026, 5, 28, 9, 0)
+
+
+def test_daily_respects_user_timezone_not_server_local():
+    """User on Pacific time, schedule says ``09:00 America/Los_Angeles``.
+    Server is UTC. At 12:00 UTC = 05:00 LA local, next run is 09:00 LA
+    today = 16:00 UTC. Pre-fix the engine used naive ``datetime.now()``
+    and read 12:00 as if it were the user's tz, mis-scheduling by the
+    server-vs-user tz offset."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('daily_time',
+                         {'time': '09:00', 'tz': 'America/Los_Angeles'},
+                         now)
+    # 09:00 LA on 2026-05-27 → 16:00 UTC (PDT, UTC-7).
+    assert result == _utc(2026, 5, 27, 16, 0)
+
+
+def test_daily_falls_back_to_default_tz_when_config_missing():
+    """``tz`` field absent on the config — pulls from ``default_tz``
+    (typically the app-level setting)."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('daily_time', {'time': '09:00'}, now,
+                         default_tz='America/Los_Angeles')
+    assert result == _utc(2026, 5, 27, 16, 0)
+
+
+def test_daily_garbage_time_string_defaults_to_midnight():
+    """Bad ``time`` string → defaults to 00:00 (engine's existing
+    behaviour). Better than crashing the scheduler when a row's
+    config was hand-edited."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('daily_time', {'time': 'garbage', 'tz': 'UTC'}, now)
+    # 00:00 today already passed → tomorrow at 00:00.
+    assert result == _utc(2026, 5, 28, 0, 0)
+
+
+def test_daily_unknown_tz_falls_back_to_utc():
+    """Unknown IANA tz string → fall back to UTC rather than crash."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at('daily_time',
+                         {'time': '15:00', 'tz': 'Imaginary/Place'},
+                         now)
+    # Treated as UTC → next run today at 15:00 UTC.
+    assert result == _utc(2026, 5, 27, 15, 0)
+
+
+# ---------------------------------------------------------------------------
+# Weekly (``trigger_type='weekly_time'``)
+# ---------------------------------------------------------------------------
+
+
+def test_weekly_picks_next_matching_weekday():
+    """It's Wednesday and the schedule wants Mon/Wed/Fri — same day
+    qualifies if the time is still in the future."""
+    # 2026-05-27 is a Wednesday.
+    now = _utc(2026, 5, 27, 8, 0)
+    result = next_run_at('weekly_time',
+                         {'time': '14:00', 'days': ['mon', 'wed', 'fri'], 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2026, 5, 27, 14, 0)
+
+
+def test_weekly_rolls_to_next_allowed_day_when_today_passed():
+    """Wednesday 18:00 UTC, schedule wants Mon/Wed/Fri at 14:00 —
+    Wed 14:00 already passed today, next match is Friday at 14:00."""
+    now = _utc(2026, 5, 27, 18, 0)  # Wed
+    result = next_run_at('weekly_time',
+                         {'time': '14:00', 'days': ['mon', 'wed', 'fri'], 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2026, 5, 29, 14, 0)  # Fri
+
+
+def test_weekly_wraps_to_next_week():
+    """Sunday past the time, schedule wants only Monday — next match
+    is the very next day."""
+    # 2026-05-31 is a Sunday.
+    now = _utc(2026, 5, 31, 15, 0)
+    result = next_run_at('weekly_time',
+                         {'time': '09:00', 'days': ['mon'], 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2026, 6, 1, 9, 0)  # next Monday
+
+
+def test_weekly_empty_days_means_every_day():
+    """Empty ``days`` list → treat as every weekday. Matches the
+    engine's existing fallback in ``_next_weekly_occurrence``."""
+    now = _utc(2026, 5, 27, 8, 0)
+    result = next_run_at('weekly_time',
+                         {'time': '14:00', 'days': [], 'tz': 'UTC'},
+                         now)
+    # Today (Wed) qualifies since 14:00 is still future.
+    assert result == _utc(2026, 5, 27, 14, 0)
+
+
+def test_weekly_unrecognised_day_abbreviations_dropped():
+    """``'mond'`` / ``'frid'`` are not in the map — silently drop.
+    If ALL listed days are invalid, fall through to the every-day
+    default (matches the empty-list behaviour)."""
+    now = _utc(2026, 5, 27, 8, 0)
+    result = next_run_at('weekly_time',
+                         {'time': '14:00', 'days': ['mond', 'frid'], 'tz': 'UTC'},
+                         now)
+    # All garbage → every day → today (Wed) qualifies.
+    assert result == _utc(2026, 5, 27, 14, 0)
+
+
+def test_weekly_day_abbreviations_case_insensitive():
+    """``MON`` / ``Mon`` / ``mon`` all parse to weekday 0."""
+    now = _utc(2026, 5, 27, 8, 0)  # Wed
+    result = next_run_at('weekly_time',
+                         {'time': '14:00', 'days': ['MON', 'WED'], 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2026, 5, 27, 14, 0)
+
+
+def test_weekly_respects_user_tz_across_day_boundary():
+    """It's 23:30 UTC on Wednesday → 16:30 LA local (still Wed).
+    Schedule fires Mon/Wed/Fri at 18:00 LA. Next run is 18:00 LA
+    today (Wed in LA, but Thursday in UTC because of the 7h offset)."""
+    now = _utc(2026, 5, 27, 23, 30)  # Wed 23:30 UTC / Wed 16:30 LA
+    result = next_run_at('weekly_time',
+                         {'time': '18:00', 'days': ['mon', 'wed', 'fri'],
+                          'tz': 'America/Los_Angeles'},
+                         now)
+    # 2026-05-27 18:00 LA → 2026-05-28 01:00 UTC.
+    assert result == _utc(2026, 5, 28, 1, 0)
+
+
+# ---------------------------------------------------------------------------
+# Monthly (``trigger_type='monthly_time'`` — NEW in PR 1)
+# ---------------------------------------------------------------------------
+
+
+def test_monthly_picks_target_day_this_month_when_future():
+    """It's the 5th, schedule fires on the 15th — next run is the
+    15th of the current month."""
+    now = _utc(2026, 5, 5, 12, 0)
+    result = next_run_at('monthly_time',
+                         {'time': '09:00', 'day_of_month': 15, 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2026, 5, 15, 9, 0)
+
+
+def test_monthly_rolls_to_next_month_when_target_day_passed():
+    """It's the 20th, schedule fires on the 15th — already past in
+    May, next run is June 15."""
+    now = _utc(2026, 5, 20, 12, 0)
+    result = next_run_at('monthly_time',
+                         {'time': '09:00', 'day_of_month': 15, 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2026, 6, 15, 9, 0)
+
+
+def test_monthly_clamps_to_last_day_when_month_too_short():
+    """Schedule wants day 31; February has 28 (or 29). Clamp to the
+    LAST valid day of that month — running a day or two early in
+    short months is less surprising than silently skipping a month
+    entirely. Standard cron convention."""
+    now = _utc(2026, 2, 1, 12, 0)  # 2026 is not a leap year
+    result = next_run_at('monthly_time',
+                         {'time': '09:00', 'day_of_month': 31, 'tz': 'UTC'},
+                         now)
+    # 2026 Feb has 28 days → run on the 28th instead.
+    assert result == _utc(2026, 2, 28, 9, 0)
+
+
+def test_monthly_handles_leap_year_february():
+    """2024 was a leap year — February has 29 days, so day-31 clamps
+    to the 29th, not the 28th."""
+    now = _utc(2024, 2, 1, 12, 0)
+    result = next_run_at('monthly_time',
+                         {'time': '09:00', 'day_of_month': 31, 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2024, 2, 29, 9, 0)
+
+
+def test_monthly_clamps_day_above_31_and_below_1():
+    """Defensive — config values outside [1, 31] clamp to the nearest
+    valid bound rather than crashing the scheduler."""
+    now = _utc(2026, 5, 5, 12, 0)
+    high = next_run_at('monthly_time',
+                      {'time': '09:00', 'day_of_month': 99, 'tz': 'UTC'},
+                      now)
+    low = next_run_at('monthly_time',
+                     {'time': '09:00', 'day_of_month': -5, 'tz': 'UTC'},
+                     now)
+    # 99 → clamped to 31 → May has 31 days → May 31st.
+    assert high == _utc(2026, 5, 31, 9, 0)
+    # -5 → clamped to 1 → next 1st is June 1 (May 1 already passed).
+    assert low == _utc(2026, 6, 1, 9, 0)
+
+
+def test_monthly_rolls_year_at_december_to_january():
+    """December 20, schedule fires on the 5th — next run is January 5
+    of the FOLLOWING year, not month 13 of the current year."""
+    now = _utc(2026, 12, 20, 12, 0)
+    result = next_run_at('monthly_time',
+                         {'time': '09:00', 'day_of_month': 5, 'tz': 'UTC'},
+                         now)
+    assert result == _utc(2027, 1, 5, 9, 0)
+
+
+def test_monthly_respects_user_tz():
+    """Schedule wants the 1st of each month at 02:00 LA. ``now`` is
+    May 1 at 06:00 UTC = April 30 at 23:00 LA. So locally we haven't
+    hit May 1 02:00 LA yet → next run is May 1 02:00 LA = May 1 09:00
+    UTC (PDT, UTC-7)."""
+    now = _utc(2026, 5, 1, 6, 0)
+    result = next_run_at('monthly_time',
+                         {'time': '02:00', 'day_of_month': 1,
+                          'tz': 'America/Los_Angeles'},
+                         now)
+    assert result == _utc(2026, 5, 1, 9, 0)
+
+
+# ---------------------------------------------------------------------------
+# Result shape — every returned datetime must be aware UTC so the engine
+# can serialise it to the DB ``next_run`` column without ambiguity.
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize('trigger_type,config', [
+    ('schedule',     {'interval': 1, 'unit': 'hours'}),
+    ('daily_time',   {'time': '09:00', 'tz': 'America/Los_Angeles'}),
+    ('weekly_time',  {'time': '09:00', 'days': ['mon'], 'tz': 'America/Los_Angeles'}),
+    ('monthly_time', {'time': '09:00', 'day_of_month': 15, 'tz': 'America/Los_Angeles'}),
+])
+def test_result_is_always_aware_utc(trigger_type, config):
+    """Engine writes the result as a naive string to the DB but the
+    convention is "stored as UTC". Returning a naive datetime would
+    leak the caller's local tz into the column. Pin the contract:
+    every result has ``tzinfo`` and is at UTC offset zero."""
+    now = _utc(2026, 5, 27, 12, 0)
+    result = next_run_at(trigger_type, config, now)
+    assert result is not None
+    assert result.tzinfo is not None
+    assert result.utcoffset() == timedelta(0)
+
+
+def test_naive_now_utc_is_coerced_to_aware_utc():
+    """Defensive — naive ``now_utc`` inputs are assumed UTC and the
+    result is still aware UTC. Matches the engine's convention
+    when parsing the DB ``next_run`` column."""
+    naive_now = datetime(2026, 5, 27, 12, 0)
+    result = next_run_at('schedule', {'interval': 1, 'unit': 'hours'}, naive_now)
+    assert result == _utc(2026, 5, 27, 13, 0)
+    assert result.tzinfo is not None