Time-zone edge cases

Name: Time-zone edge cases (tz-edge-cases corpus)
Creator: tz-edge-cases
License: https://creativecommons.org/publicdomain/zero/1.0/

68 cases where converting a time is not the arithmetic people expect — local times that never happened, local times that happened twice, offsets that are not a whole number of hours, calendar dates that a country skipped entirely, transitions that moved no clocks at all, and zones where daylight saving is the lower offset.

This is the corpus this site's own time-zone logic is tested against. It is published here because the cases are the useful artefact: each one names a zone, an instant, and the value a correct implementation produces, so it can be read as a reference or lifted directly into someone else's test suite.

64 of the 68 cases are frozen — they concern instants in the past, so no future release of the time-zone database can change them. The remaining 4 are marked volatile and re-verified on every release: 2 because they depend on rules a government could still change, and 2 because they concern pre-1970 local time, where the main database and its backzone file legitimately disagree.

Times that never happened

10 cases in the `gap` category of the tz-edge-cases corpus — local times that do not exist — with the value each one is expected to produce.
Case	Zone	Local time asked for	Gap	Shifting back / forward gives	Stability
`gap-cairo-midnight-2023` Egypt re-adopted DST in 2023 and transitions at midnight, so 28 April has no 00:xx hour: the local day starts at 01:00.	`Africa/Cairo`	2023-04-28T00:30:00	2023-04-28T00:00:00 → 2023-04-28T01:00:00 60 minutes	2023-04-27T23:30:00+02:00 2023-04-28T01:30:00+03:00	frozen
`gap-casablanca-ramadan-end-2024` Morocco returns to +01:00 after Ramadan, creating a gap.	`Africa/Casablanca`	2024-04-14T02:30:00	2024-04-14T02:00:00 → 2024-04-14T03:00:00 60 minutes	2024-04-14T01:30:00+00:00 2024-04-14T03:30:00+01:00	frozen
`gap-casey-3hour-2020` Casey shifts by three hours, the largest routine jump in the database — larger than Troll's two.	`Antarctica/Casey`	2020-10-04T01:30:00	2020-10-04T00:01:00.644531 → 2020-10-04T03:01:00.644531 180 minutes	2020-10-03T22:30:00+08:00 2020-10-04T04:30:00+11:00	frozen
`gap-chatham-2024` Chatham transitions at 02:45 from a :45 base offset.	`Pacific/Chatham`	2024-09-29T03:00:00	2024-09-29T02:45:00 → 2024-09-29T03:45:00 60 minutes	2024-09-29T02:00:00+12:45 2024-09-29T04:00:00+13:45	frozen
`gap-lordhowe-30min-2024` Lord Howe Island shifts by 30 minutes, not 60.	`Australia/Lord_Howe`	2024-10-06T02:15:00	2024-10-06T02:00:00 → 2024-10-06T02:30:00 30 minutes	2024-10-06T01:45:00+10:30 2024-10-06T02:45:00+11:00	frozen
`gap-santiago-southern-2024` Southern hemisphere: spring forward occurs in September.	`America/Santiago`	2024-09-08T00:30:00	2024-09-08T00:00:00 → 2024-09-08T01:00:00 60 minutes	2024-09-07T23:30:00-04:00 2024-09-08T01:30:00-03:00	frozen
`gap-troll-2hr-2024` Troll station shifts by 2 hours, producing a 2-hour gap.	`Antarctica/Troll`	2024-03-31T01:30:00	2024-03-31T01:00:00 → 2024-03-31T03:00:00 120 minutes	2024-03-30T23:30:00+00:00 2024-03-31T03:30:00+02:00	frozen
`gap-uk-2024` UK transitions at 01:00 local, not 02:00 — off-by-one-hour trap.	`Europe/London`	2024-03-31T01:30:00	2024-03-31T01:00:00 → 2024-03-31T02:00:00 60 minutes	2024-03-31T00:30:00+00:00 2024-03-31T02:30:00+01:00	frozen
`gap-us-eastern-2024` Northern-hemisphere 60-minute spring forward; 02:30 never occurs.	`America/New_York`	2024-03-10T02:30:00	2024-03-10T02:00:00 → 2024-03-10T03:00:00 60 minutes	2024-03-10T01:30:00-05:00 2024-03-10T03:30:00-04:00	frozen
`volatile-us-eastern-2027-gap` Correct under current US rules; would change if permanent DST passes.	`America/New_York`	2027-03-14T02:30:00	2027-03-14T02:00:00 → 2027-03-14T03:00:00 60 minutes	2027-03-14T01:30:00-05:00 2027-03-14T03:30:00-04:00	volatile — future rules

Times that happened twice

6 cases in the `ambiguous` category of the tz-edge-cases corpus — local times that occur twice — with the value each one is expected to produce.
Case	Zone	Local time asked for	First occurrence	Second occurrence	Stability
`ambiguous-auckland-2024` Southern hemisphere: fall back occurs in April.	`Pacific/Auckland`	2024-04-07T02:30:00	{'instantUtc': '2024-04-06T13:30:00+00:00', 'utcOffset': '+13:00', 'abbreviation': 'NZDT', 'isDst': True}	{'instantUtc': '2024-04-06T14:30:00+00:00', 'utcOffset': '+12:00', 'abbreviation': 'NZST', 'isDst': False}	frozen
`ambiguous-berlin-2024` EU-wide transition at 03:00 local; 02:30 occurs twice.	`Europe/Berlin`	2024-10-27T02:30:00	{'instantUtc': '2024-10-27T00:30:00+00:00', 'utcOffset': '+02:00', 'abbreviation': 'CEST', 'isDst': True}	{'instantUtc': '2024-10-27T01:30:00+00:00', 'utcOffset': '+01:00', 'abbreviation': 'CET', 'isDst': False}	frozen
`ambiguous-casablanca-ramadan-start-2024` Morocco drops to +00:00 for Ramadan — an inverted DST fall-back.	`Africa/Casablanca`	2024-03-10T02:30:00	{'instantUtc': '2024-03-10T01:30:00+00:00', 'utcOffset': '+01:00', 'abbreviation': '+01', 'isDst': False}	{'instantUtc': '2024-03-10T02:30:00+00:00', 'utcOffset': '+00:00', 'abbreviation': '+00', 'isDst': True}	frozen
`ambiguous-lordhowe-30min-2024` 30-minute ambiguity window rather than the usual 60.	`Australia/Lord_Howe`	2024-04-07T01:45:00	{'instantUtc': '2024-04-06T14:45:00+00:00', 'utcOffset': '+11:00', 'abbreviation': '+11', 'isDst': True}	{'instantUtc': '2024-04-06T15:15:00+00:00', 'utcOffset': '+10:30', 'abbreviation': '+1030', 'isDst': False}	frozen
`ambiguous-santiago-2024` Chile transitions at 24:00, so the ambiguous window is late evening.	`America/Santiago`	2024-04-06T23:30:00	{'instantUtc': '2024-04-07T02:30:00+00:00', 'utcOffset': '-03:00', 'abbreviation': '-03', 'isDst': True}	{'instantUtc': '2024-04-07T03:30:00+00:00', 'utcOffset': '-04:00', 'abbreviation': '-04', 'isDst': False}	frozen
`ambiguous-us-eastern-2024` 01:30 occurs twice; EDT then EST.	`America/New_York`	2024-11-03T01:30:00	{'instantUtc': '2024-11-03T05:30:00+00:00', 'utcOffset': '-04:00', 'abbreviation': 'EDT', 'isDst': True}	{'instantUtc': '2024-11-03T06:30:00+00:00', 'utcOffset': '-05:00', 'abbreviation': 'EST', 'isDst': False}	frozen

Calendar dates a place skipped

1 case in the `skipped_date` category of the tz-edge-cases corpus — local times that do not exist — with the value each one is expected to produce.
Case	Zone	Local time asked for	Gap	Shifting back / forward gives	Stability
`skipped-apia-2011-12-30` Samoa skipped 30 Dec 2011 entirely when crossing the date line.	`Pacific/Apia`	2011-12-30T12:00:00	2011-12-30T00:00:00 → 2011-12-31T00:00:00 1440 minutes	2011-12-29T12:00:00-10:00 2011-12-31T12:00:00+14:00	frozen

2 cases in the `skipped_date` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`skipped-apia-after` Day after the skip: +14:00, a 24-hour jump.	`Pacific/Apia`	2011-12-31T12:00:00	+14:00	+14	in effect	frozen
`skipped-apia-before` Day before the skip: -10:00.	`Pacific/Apia`	2011-12-29T12:00:00	-10:00	-10	in effect	frozen

Rule changes and abolitions

7 cases in the `rule_change` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`rulechange-istanbul-2015-winter` Istanbul in January 2015, still alternating: EET +02:00.	`Europe/Istanbul`	2015-01-15T12:00:00	+02:00	EET	not in effect	frozen
`rulechange-istanbul-2017-winter` January 2017: +03:00. Abolishing DST left the clocks an hour forward of where they used to be in winter — the opposite of Iran and Mexico, which settled on their standard offset.	`Europe/Istanbul`	2017-01-15T12:00:00	+03:00	+03	not in effect	frozen
`rulechange-juarez-2023-still-dst` Mexican border municipalities kept US DST rules after 2022.	`America/Ciudad_Juarez`	2023-06-15T12:00:00	-06:00	MDT	in effect	frozen
`rulechange-mazatlan-2021-dst` Mexico observed DST in 2021.	`America/Mazatlan`	2021-06-15T12:00:00	-06:00	MDT	in effect	frozen
`rulechange-mazatlan-2023-no-dst` Mexico abolished DST for most of the country in 2022.	`America/Mazatlan`	2023-06-15T12:00:00	-07:00	MST	not in effect	frozen
`rulechange-tehran-2021-dst` Iran observed DST (+04:30) in 2021.	`Asia/Tehran`	2021-06-15T12:00:00	+04:30	+0430	in effect	frozen
`rulechange-tehran-2023-no-dst` Iran abolished DST in 2022; same date is now +03:30.	`Asia/Tehran`	2023-06-15T12:00:00	+03:30	+0330	not in effect	frozen

Transitions that moved no clocks

6 cases in the `zero_offset_transition` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`zerooffset-asuncion-2024-dst` Paraguay began daylight saving as usual in October 2024: -03:00, DST set.	`America/Asuncion`	2024-10-10T12:00:00	-03:00	-03	in effect	frozen
`zerooffset-asuncion-2024-permanent` Nine days later Paraguay made that offset permanent. Same -03:00, DST flag clear, no clock movement.	`America/Asuncion`	2024-10-20T12:00:00	-03:00	-03	not in effect	frozen
`zerooffset-guam-2000-gst` Guam as GST, +10:00, no DST.	`Pacific/Guam`	2000-12-01T12:00:00	+10:00	GST	not in effect	frozen
`zerooffset-guam-2001-chst` Renamed to ChST. Offset and DST flag both unchanged — the abbreviation is the only thing that moved.	`Pacific/Guam`	2001-01-15T12:00:00	+10:00	ChST	not in effect	frozen
`zerooffset-istanbul-2016-dst` Türkiye's last summer of daylight saving: +03:00, DST flag set.	`Europe/Istanbul`	2016-07-15T12:00:00	+03:00	EEST	in effect	frozen
`zerooffset-istanbul-2017-permanent` The same +03:00 a year later, DST flag now clear and the abbreviation changed EEST to +03. The September 2016 transition moved no clocks at all.	`Europe/Istanbul`	2017-07-15T12:00:00	+03:00	+03	not in effect	frozen

Southern-hemisphere and inverted DST

4 cases in the `inverted_dst` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`inverted-casablanca-after-ramadan-2024` Morocco back at +01:00 after Ramadan.	`Africa/Casablanca`	2024-04-20T12:00:00	+01:00	+01	not in effect	frozen
`inverted-casablanca-before-ramadan-2024` Morocco at its normal +01:00 before Ramadan.	`Africa/Casablanca`	2024-03-01T12:00:00	+01:00	+01	not in effect	frozen
`inverted-casablanca-during-ramadan-2024` Morocco at +00:00 during Ramadan — offset DROPS, on a lunar date.	`Africa/Casablanca`	2024-03-15T12:00:00	+00:00	+00	in effect	frozen
`volatile-casablanca-2027` Depends on the projected lunar Ramadan window for 2027.	`Africa/Casablanca`	2027-06-15T12:00:00	+00:00	+00	not in effect	volatile — future rules

Where daylight saving is the lower offset

4 cases in the `negative_dst` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`negativedst-dublin-january` Irish Standard Time is the SUMMER offset. Winter GMT is modelled as negative daylight saving, so isDst is true in January at +00:00.	`Europe/Dublin`	2024-01-15T12:00:00	+00:00	GMT	in effect	frozen
`negativedst-dublin-july` July is +01:00 IST with isDst false — inverted relative to every other EU zone, which flag their summer offset as DST.	`Europe/Dublin`	2024-07-15T12:00:00	+01:00	IST	not in effect	frozen
`negativedst-windhoek-2016-january` Namibia before 2017: January is CAT +02:00 with isDst false.	`Africa/Windhoek`	2016-01-15T12:00:00	+02:00	CAT	not in effect	frozen
`negativedst-windhoek-2016-june` June is WAT +01:00 with isDst TRUE. The winter offset carries the DST flag, and 'West Africa Time' is the daylight-saving name.	`Africa/Windhoek`	2016-06-15T12:00:00	+01:00	WAT	in effect	frozen

Zones that changed their standard offset

6 cases in the `offset_move` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`offsetmove-almaty-2024-after` +05:00 afterwards. The transition fell on 29 February — a standard-offset change, no DST anywhere near it, on a leap day.	`Asia/Almaty`	2024-03-15T12:00:00	+05:00	+05	not in effect	frozen
`offsetmove-almaty-2024-before` Kazakhstan at +06:00 before unifying its two zones.	`Asia/Almaty`	2024-02-15T12:00:00	+06:00	+06	not in effect	frozen
`offsetmove-caracas-2016-after` Retired to -04:00 in May 2016. A hardcoded offset table written before then is still wrong today.	`America/Caracas`	2016-06-15T12:00:00	-04:00	-04	not in effect	frozen
`offsetmove-caracas-2016-before` Venezuela at -04:30, a sub-hour STANDARD offset.	`America/Caracas`	2016-04-15T12:00:00	-04:30	-0430	not in effect	frozen
`offsetmove-famagusta-2017-plus03` Northern Cyprus followed Türkiye to permanent +03:00 in 2016.	`Asia/Famagusta`	2017-01-15T12:00:00	+03:00	+03	not in effect	frozen
`offsetmove-famagusta-2018-reverted` Then reverted to EET +02:00 in October 2017 — the same decision taken and untaken thirteen months apart, while Istanbul kept +03:00.	`Asia/Famagusta`	2018-01-15T12:00:00	+02:00	EET	not in effect	frozen

Offsets that are not a whole hour

5 cases in the `sub_hour_offset` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`offset-eucla-0845` +08:45, one of the rarest offsets in use.	`Australia/Eucla`	2024-06-15T09:00:00	+08:45	+0845	not in effect	frozen
`offset-kathmandu-0545` +05:45 base offset; breaks integer-hour offset models.	`Asia/Kathmandu`	2024-06-15T09:00:00	+05:45	+0545	not in effect	frozen
`offset-kolkata-0530` +05:30 base offset, no DST.	`Asia/Kolkata`	2024-06-15T09:00:00	+05:30	IST	not in effect	frozen
`offset-marquesas-neg0930` -09:30; negative sub-hour offset, a common sign-handling bug.	`Pacific/Marquesas`	2024-06-15T09:00:00	-09:30	-0930	not in effect	frozen
`offset-monrovia-1971-neg004430` Liberia kept -00:44:30 by law until January 1972: a legal offset with a seconds component. Unlike the pre-1970 LMT cases it is frozen, so a mismatch here is a real bug.	`Africa/Monrovia`	1971-06-15T12:00:00	-00:44:30	MMT	not in effect	frozen

One country, more than one answer

4 cases in the `intra_country` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`intracountry-brisbane-january` Brisbane does not observe DST (+10:00) — same country, same instant.	`Australia/Brisbane`	2024-01-15T12:00:00	+10:00	AEST	not in effect	frozen
`intracountry-denver-july` Denver does (-06:00), despite sharing a nominal zone name.	`America/Denver`	2024-07-01T12:00:00	-06:00	MDT	in effect	frozen
`intracountry-phoenix-july` Arizona does not observe DST (-07:00).	`America/Phoenix`	2024-07-01T12:00:00	-07:00	MST	not in effect	frozen
`intracountry-sydney-january` Sydney observes DST in January (+11:00).	`Australia/Sydney`	2024-01-15T12:00:00	+11:00	AEDT	in effect	frozen

Before standard time

2 cases in the `lmt` category of the tz-edge-cases corpus — single-instant offset resolutions — with the value each one is expected to produce.
Case	Zone	Local time	UTC offset	Abbreviation	Daylight saving	Stability
`lmt-amsterdam-1890` Offset is not a whole number of minutes. Also build-dependent: upstream merges pre-1970 Amsterdam into Brussels (+00:17:30 BMT), while backzone builds keep +00:19:32 AMT.	`Europe/Amsterdam`	1890-06-15T12:00:00	+00:17:30	BMT	not in effect	volatile — pre 1970 build variance
`lmt-kolkata-1880` Madras Mean Time, +05:21:10. Agrees across common builds today, but pre-1970 data carries no stability guarantee.	`Asia/Kolkata`	1880-06-15T12:00:00	+05:21:10	MMT	not in effect	volatile — pre 1970 build variance

Conversions between two zones

11 cases in the `conversion` category of the tz-edge-cases corpus — conversions between two zones — with the value each one is expected to produce.
Case	From	To	Result	Offset change	Stability
`conv-ambiguous-source-fold0` Ambiguous source time, first occurrence (EDT).	`America/New_York` 2024-11-03T01:30:00	`UTC`	2024-11-03T05:30:00 same calendar day	-04:00 → +00:00 +240 minutes	frozen
`conv-ambiguous-source-fold1` Same wall clock, second occurrence (EST) — one hour later in UTC.	`America/New_York` 2024-11-03T01:30:00	`UTC`	2024-11-03T06:30:00 same calendar day	-05:00 → +00:00 +300 minutes	frozen
`conv-apia-la-datechange` Near the date line: same instant, previous calendar day at the target.	`Pacific/Apia` 2024-06-15T10:00:00	`America/Los_Angeles`	2024-06-14T14:00:00 -1 calendar day	+13:00 → -07:00 -1200 minutes	frozen
`conv-istanbul-athens-divergence` Istanbul and Athens shared EET/EEST until September 2016. Afterwards they differ by an hour in winter and not at all in summer.	`Europe/Istanbul` 2017-01-15T12:00:00	`Europe/Athens`	2017-01-15T11:00:00 same calendar day	+03:00 → +02:00 -60 minutes	frozen
`conv-kathmandu-kolkata-15min` Neighbouring zones 15 minutes apart (+05:45 to +05:30).	`Asia/Kathmandu` 2024-06-15T09:00:00	`Asia/Kolkata`	2024-06-15T08:45:00 same calendar day	+05:45 → +05:30 -15 minutes	frozen
`conv-kiritimati-midway-25h` +14:00 to -11:00: the widest spread in use, 25 hours, landing on the previous calendar day.	`Pacific/Kiritimati` 2024-06-15T10:00:00	`Pacific/Midway`	2024-06-14T09:00:00 -1 calendar day	+14:00 → -11:00 -1500 minutes	frozen
`conv-kolkata-eucla-315` Delta of 3h15m: neither endpoint nor the difference is a whole hour.	`Asia/Kolkata` 2024-06-15T09:00:00	`Australia/Eucla`	2024-06-15T12:15:00 same calendar day	+05:30 → +08:45 +195 minutes	frozen
`conv-kolkata-newyork-dayshift` Conversion moves backwards across the date boundary.	`Asia/Kolkata` 2024-06-15T02:00:00	`America/New_York`	2024-06-14T16:30:00 -1 calendar day	+05:30 → -04:00 -570 minutes	frozen
`conv-ny-london-desync-march` US has sprung forward, UK has not: the gap is 4 hours, not the usual 5.	`America/New_York` 2024-03-10T09:00:00	`Europe/London`	2024-03-10T13:00:00 same calendar day	-04:00 → +00:00 +240 minutes	frozen
`conv-ny-london-desync-october` UK has fallen back, US has not: again 4 hours, in the other direction.	`America/New_York` 2024-10-27T09:00:00	`Europe/London`	2024-10-27T13:00:00 same calendar day	-04:00 → +00:00 +240 minutes	frozen
`conv-ny-london-normal` Baseline: the usual 5-hour gap, for contrast with the two above.	`America/New_York` 2024-06-15T09:00:00	`Europe/London`	2024-06-15T14:00:00 same calendar day	-04:00 → +01:00 +300 minutes	frozen

Where this comes from

The corpus is tz-edge-cases, version 1.1.0, generated against IANA time-zone database release 2026c and released into the public domain under CC0. This site vendors a pinned copy; the machine-readable form is fixtures.json and its JSON Schema.

The values in these tables are the corpus's own expected values, printed as they are stored. They are not recomputed for display — and they do not need to be, because this build has already checked them: every frozen case is compared against the build's own time-zone data (release 2026b) before any page is generated, and a single mismatch fails the build rather than publishing a table.

That check covers the frozen cases, and the honest limit is worth stating twice over. It shows that release 2026b agrees with the corpus, not that either is right about the world. And it does not cover the 4 volatile cases: the 2 that depend on future rules cannot be checked against anything yet, and the 2 pre-1970 ones this build can actively disagree with — the corpus was generated against the main database and this build resolves those instants a couple of minutes differently, which is the documented disagreement between tzdb and its backzone file rather than an error in either. Every volatile case is marked as one in the tables above.

For what each zone does across a whole year rather than at a single instant, see daylight saving time by zone.