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mejbaurbaharmejbaurbahar/software-tester-skills/skills/date-time-timezone-testing/SKILL.md

date-time-timezone-testing

Skill1 starsChanged 53 days agoMITView on GitHub

Use when testing anything involving dates, times, time zones, DST, leap years/seconds, scheduling, expiry, recurring events, cron jobs, or locale calendars — with frozen clocks, boundary calendars, and the classic time bugs checklist.

The file

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  1. Very specific:

    Compare instants, not strings: Use ISO-8601 with offset; avoid Date parsing of ambiguous 03/04/2026.

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Files
SKILL.md

Date, Time & Time-Zone Testing

Time bugs are silent, seasonal and expensive. Make time an injectable dependency and test the calendar's sharp edges deliberately.

Test-date matrix (always include)

CaseExample values
Leap day2024-02-29, 2023-02-28→03-01, 2100-02-29 invalid (century rule)
Month/year endsJan 31 + 1 month, Dec 31 23:59:59 → Jan 1, ISO week 53
DST spring-forward2026-03-08 02:30 America/New_York (does not exist)
DST fall-back2026-11-01 01:30 (occurs twice)
Odd offsetsAsia/Kolkata +05:30, Asia/Kathmandu +05:45, Australia/Lord_Howe 30-min DST
Zone changesuser travels; server UTC vs browser local; zone rule updates (tzdata)
Epoch limits32-bit overflow 2038-01-19, negative epochs, 0
Locale calendarsBuddhist/Hijri/Japanese eras, week starting Sunday vs Monday
Leap second23:59:60 tolerated by parsers

Techniques

  1. Freeze the clock: freezegun/time-machine (Py), jest.useFakeTimers().setSystemTime(), Clock.fixed() (Java), faketime/libfaketime for binaries, TZ=Asia/Dhaka env var.
  2. Run the suite under multiple zones: for tz in UTC America/Los_Angeles Asia/Kolkata Pacific/Auckland; do TZ=$tz npm test; done.
  3. Store UTC, display local; store the zone ID (not offset) for future events (meetings, reminders).
  4. Compare instants, not strings. Use ISO-8601 with offset; avoid Date parsing of ambiguous 03/04/2026.
  5. Property test: parse(format(t)) == t for random instants across zones.
  6. Scheduler tests: advance a fake clock past triggers; verify missed-run and double-run behavior across DST; cron in UTC.
  7. Expiry logic: assert at expiry-1s, expiry, expiry+1s; token TTL and clock skew tolerance (±60s).
from freezegun import freeze_time
@freeze_time("2026-03-08 06:59:59", tz_offset=0)   # 1s before US DST jump
def test_reminder_survives_dst(): ...

Bug checklist

new Date("2026-01-31") parsed as UTC then displayed a day earlier · + 24h ≠ next day on DST · month arithmetic overflow · week-year (YYYY vs yyyy) in formatters · DB TIMESTAMP vs TIMESTAMPTZ · naive vs aware datetimes · sorting by string date · server local time in cron · "end of day" excluded from ranges.

Related

localization-testing, unit-testing, property-based-testing, database-testing

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