BST vs GMT

BST vs GMT side by side

Last verified October 2, 2026. Offsets and dates are standard U.S. and U.K. rules; the tool reads live clock data.

When each one applies

Write BST for U.K. dates in the summer half and GMT for the winter half. Many people write GMT all year, which is wrong from March to October.

U.K. and U.S. clocks do not change on the same dates: the U.S. changes earlier in March and later in November. For a few weeks each year the U.K.-to-U.S. gap is one hour smaller than usual, so recheck meeting times in those weeks.

If you are unsure, say "U.K. time" or "London time" and let the reader work it out.

Exceptions: Arizona, Hawaii and split states

• Arizona (except the Navajo Nation) stays on Mountain Standard Time all year and never changes its clocks, so Arizona matches Pacific Daylight Time in summer and Mountain Daylight Time only in winter.
• Hawaii stays on Hawaii Standard Time (UTC−10) all year. The Aleutian Islands of Alaska, west of 169°30′ W, use Hawaii–Aleutian time and do change clocks.
• Puerto Rico and the U.S. Virgin Islands stay on Atlantic Standard Time (UTC−4) all year, so they match Eastern Daylight Time in summer and are an hour ahead of Eastern Standard Time in winter.
• States split across two zones: Alaska, Florida, Idaho, Indiana, Kansas, Kentucky, Michigan, Nebraska, North Dakota, Oregon, South Dakota, Tennessee and Texas. Check the county or city, not just the state.

How this works

Pick two zones and the tool reads your device's clock and your browser's built-in time zone data (the IANA database), so there is no outside data service and the answer stays right when the clocks change. Named zones such as Eastern or Pacific follow the clock; abbreviations such as EST or PDT are fixed offsets from UTC.

The hour difference compares both offsets for today's date. The converter shifts a time from the first zone to the second and flags the next or previous day. The business-hours overlap finds the window when 9 a.m. to 5 p.m. in one zone meets 9 a.m. to 5 p.m. in the other.

The clock-change dates are found by checking each day ahead for a change in the zone's offset, so they come from the data and not from a hard-coded rule.

Where these numbers come from

The figures and defaults on this page are based on:

• NIST Time and Frequency Division: daylight saving time, US rules (second Sunday of March to first Sunday of November) and the 2026 dates, March 8 to November 1 (checked October 2026)
• IANA: Time Zone Database, the reference data browsers and operating systems use for zone offsets and clock-change rules (checked October 2026)

Clock rules can change by law. The tool uses your browser's time zone data; check an official source for anything legal, travel or safety related.