EST vs EDT, PST vs PDT: Why the Abbreviation Changes in Summer
EST and PST are winter abbreviations. From March through November, the same US regions switch to EDT and PDT. A converter that always treats EST as UTC−5 can be off by exactly one hour for most of the year.
Quick Reference
| Abbr | Full Name | Offset | Active | Cities |
|---|---|---|---|---|
| EST | Eastern Standard Time | UTC−5 | Nov – Mar | New York, Toronto, Washington DC |
| EDT | Eastern Daylight Time | UTC−4 | Mar – Nov | Same cities — clocks sprung forward |
| PST | Pacific Standard Time | UTC−8 | Nov – Mar | Los Angeles, San Francisco, Seattle |
| PDT | Pacific Daylight Time | UTC−7 | Mar – Nov | Same cities — clocks sprung forward |
| GMT | Greenwich Mean Time | UTC±0 | Oct – Mar | London in winter |
| BST | British Summer Time | UTC+1 | Mar – Oct | London in summer |
EST vs EDT
EST (Eastern Standard Time) is UTC−5. It applies to US eastern states from the first Sunday of November through the second Sunday of March — roughly five months of the year.
EDT (Eastern Daylight Time) is UTC−4. It applies during the remaining seven months, when clocks spring forward by one hour. New York, Boston, Toronto, Washington DC, and Miami all switch between EST and EDT on the same dates.
Many people use "EST" year-round as a shorthand, but saying "3 PM EST" in July technically means 3 PM UTC−5 — one hour behind where New York actually is. For scheduling, use "ET" (Eastern Time) if you want a label that is correct in all seasons, or use the IANA identifier America/New_York in any system that accepts it.
PST vs PDT
PST (Pacific Standard Time) is UTC−8, active from November through March in Los Angeles, San Francisco, Seattle, Portland, and Vancouver.
PDT (Pacific Daylight Time) is UTC−7, active March through November. The switch happens on the same dates as the eastern US. Both coasts spring forward and fall back together, so the EST–PST gap (3 hours) is the same as the EDT–PDT gap (3 hours).
Like "ET" for the east coast, "PT" (Pacific Time) is the season-agnostic label that is always correct for the US west coast.
Why Static Converters Can Be Off by One Hour
Some converter sites maintain a fixed table: EST = UTC−5, PST = UTC−8, GMT = UTC+0, and so on. This approach is fast to build but produces wrong answers for most of the calendar year.
Example: 3 PM New York in June
- ✗Fixed-table answer: 3 PM EST = 8 PM GMT (wrong — New York is on EDT in June, not EST)
- ✓IANA-based answer: 3 PM EDT = 8 PM BST (correct — both cities are on DST in June)
The error here is two layers: the converter wrongly keeps New York on EST, and also wrongly keeps London on GMT. In June, London is on BST (UTC+1). The fixed-table result is off by one hour on each side, but because both errors go in the same direction, the conversion total is still correct — 3 PM to 8 PM. However, the abbreviations shown are wrong, which confuses anyone cross-checking the result.
Where fixed-table converters cause real scheduling errors: when only one region has switched clocks. During the 2–3 week window each spring when the US has already switched to daylight time but Europe has not (or vice versa in fall), the offset between the two regions shifts by one hour from its usual value. A fixed-table converter cannot know this.
TimezoneCast calculates every offset from the IANA timezone database at the actual date and time in question — not from a static lookup table.
GMT vs BST — The Same Problem for London
London follows the same pattern. GMT (Greenwich Mean Time) is UTC+0 and applies in winter. From late March through late October, the UK switches to BST (British Summer Time), which is UTC+1.
A converter that always shows London as GMT will be one hour behind the correct answer from late March through late October. The same logic applies: use city names or IANA identifiers, not fixed abbreviations, to guarantee accuracy.
The 2–3 Week Transition Window
The US (and Canada) switch clocks on the second Sunday of March. Most of Europe switches clocks on the last Sunday of March — roughly two weeks later. A similar gap occurs in fall: the EU switches on the last Sunday of October, the US on the first Sunday of November.
During those two-to-three week gaps, the offset between a US city and a European city is one hour different from its usual value. If your team has a standing meeting at "10 AM ET / 3 PM London time," that meeting will fall one hour off during those windows unless calendar invites are expressed in UTC or anchored to IANA city identifiers.
See the guide on how Daylight Saving Time affects remote teams for a full breakdown of these transition windows.
Frequently Asked Questions
- What is the difference between EST and EDT?
- EST (Eastern Standard Time) is UTC−5 and applies from the first Sunday of November through the second Sunday of March. EDT (Eastern Daylight Time) is UTC−4 and applies during the remaining months when clocks spring forward. The same geographic region uses both abbreviations — EST in winter, EDT in summer.
- Is it EST or EDT right now?
- From the second Sunday of March through the first Sunday of November, New York and other eastern US states are on EDT (UTC−4), not EST. From November through March, they are on EST (UTC−5). Many people say 'EST' year-round, but the technically correct summer abbreviation is EDT.
- What is the difference between PST and PDT?
- PST (Pacific Standard Time) is UTC−8 and applies from the first Sunday of November through the second Sunday of March. PDT (Pacific Daylight Time) is UTC−7 and applies in summer. Los Angeles, San Francisco, Seattle, and Vancouver all switch between PST and PDT on the same dates as the eastern US switches between EST and EDT.
- Why do some converters show the wrong time for EST?
- Older or simpler converters use a fixed table where 'EST = UTC−5' always. That is only correct from November to March. From March to November, eastern US is actually on EDT (UTC−4), so a fixed-EST converter will be exactly one hour off. Accurate converters like TimezoneCast use the IANA timezone database, which knows the exact rule for every date.
- Does the EST to PST offset change with DST?
- No — EST and PST are both standard-time labels, so the gap between them is always 3 hours (UTC−5 minus UTC−8). Likewise, the gap between EDT and PDT is always 3 hours (UTC−4 minus UTC−7). However, during the 2–3 week window when one region has switched clocks and the other has not, the live offset between those two cities temporarily changes by one hour.
- What is the EST to PDT or EDT to PST situation?
- In early spring and late fall, the US and Canada switch clocks on the same date, so both regions move together. But the US and Europe switch clocks on different dates, creating a 2–3 week window where the gap between a US city and a European city is one hour different from its usual value. During those weeks, any standing meeting time in UTC or city-name terms remains correct, but abbreviation-based references can be misleading.