How does the checker know DST rules?

It uses the IANA Time Zone Database bundled with your browser. When DST policy changes (as it does periodically), updating your browser updates the rules.

Which zones don't observe DST?

Most of Asia, Africa, and parts of Australia. Arizona (except the Navajo Nation), Hawaii, and most of the equatorial world don't shift. The checker reports "no DST" for these zones.

Why does Europe shift on a different day than the US?

EU DST ends on the last Sunday of October at 01:00 UTC; US DST ends on the first Sunday of November at 02:00 local time. The two regions are misaligned for about a week each spring and fall.

Is the lookup performed locally?

Yes. The IANA data and lookup logic run in your browser; no requests are made.

DST detection · spring/fall · 2 years · live

When does DST start and end?

Pick any IANA timezone. We scan the year-by-year offset table to find every DST transition for this year and next, with direction and exact dates.

Quick Start

How DST detection works

We walk one day at a time, asking Intl for the offset on that date. Any change is a DST transition — we record direction and the before/after offsets.

01
Pick zone
Defaults to your browser's timezone.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
zoneISO 8601RFC 3339epochlive
in · timestampIANA · ISO
```
Europe/Berlin
```
loaded · ISO 8601zone · ready
02
Scan days
365 daily offset checks per year — diff against previous.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via Intl.DateTimeFormat.
scanUTC mathIntlDST-safelive
loop · options366 · live
```
for day in year
```
options applied · UTCDST-safe
03
Detect direction
Offset increased → spring forward. Decreased → fall back.
Output respects the chosen timezone offset (Z vs +05:30) and pads digits per ISO conventions so sorts stay chronological.
diroffsetzone-awaresortablepadded
Δoff · computed+/- · ready
```
+60 = spring
```
computed · instantsortable
04
List events
This year and next, with day-of-week names.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
eventsclipboardno oracledeterministicclient-side
✓ · copiedready · done
```
Sun, Mar 29
```
shipped · ready to shipsuccess

Pro tips

Time math without DST landmines

Always store UTC

Convert to local for display only. UTC storage avoids DST jumps and timezone drift in databases.

Watch leap seconds

POSIX time doesn’t count leap seconds; some systems do. Don’t use timestamps for sub-second precision across boundaries.

Prefer Temporal eventually

The Date API has many quirks. Temporal is shipping in modern engines and handles zones correctly.

Cron is timezone-tricky

Cron expressions run in the daemon’s local zone unless told otherwise. Pin a TZ explicitly for predictable runs.

ISO 8601 sorts naturally

YYYY-MM-DDTHH:mm:ss sorts correctly as a string. Don’t use US-style MM/DD/YYYY for anything machine-readable.

Beware Y2K38

Signed 32-bit Unix timestamps overflow in 2038. Use 64-bit storage end-to-end for long-lived systems.

Use Cases

When to check DST

Scheduling, ops, travel.

Calendar invites

Confirm a meeting time isn't ambiguous near the change. Math runs in UTC under the hood to dodge DST jumps — local time is only for display. That means scheduling logic and date diffs survive March / November fall-back without the classic off-by-one-hour bug.

DST-safe

Cron audits

Look for jobs that double-fire or skip on DST days. Browser clock is the only source — no external time server, no privacy leak, no rate limit. Pair with NTP-synced server time for high-accuracy use cases, but for human-scale work the browser clock is plenty.

UTC-correct

Travel

Will your destination shift before arrival? ISO 8601 output sorts naturally as strings, perfect for filenames (report-2026-01-15.pdf) and log lines that need chronological ordering without parsing. Drop into S3 keys for free time-bucketing.

ISO-sortable

Tests

Use known DST dates as test fixtures. Pairs cleanly with cron, scheduled tasks, and timestamped log analysis. The tool's output drops directly into APScheduler, Linux cron, or GitHub Actions schedule expressions without re-formatting.

spec-grade

Reminders

Re-set hardware clocks on transition Sundays. Output respects the chosen timezone offset (Z vs +05:30) so REST APIs and database TIMESTAMP WITH TIME ZONE columns round-trip cleanly. No ambiguity between "noon UTC" and "noon local".

zone-aware

Multi-region

Different countries shift on different weekends. Confirms the difference between epoch seconds (10 digits) and milliseconds (13 digits) before you commit code that mis-multiplies by 1000 — a classic bug in cross-language time handling.

epoch-accurate

FAQ

Frequently asked questions

Why no transitions?

The zone has a fixed offset (e.g. Asia/Tokyo, India). It doesn't observe DST.

How is the change detected?

Polling Intl for daily offsets and diffing — same algorithm a tzdata-aware app would use.

Does it handle southern hemisphere?

Yes — Australia and Brazil 'spring' in October and 'fall back' in April.

When does my country drop DST?

Policies change — re-check next year if there's news about reform.

Is anything sent to a server?

No. Detection is all client-side via Intl.

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