Two settings of the same US Eastern time zone. EST is Eastern Standard Time, UTC−5, used in winter. EDT is Eastern Daylight Time, UTC−4, used in summer when clocks go forward. If you are not sure which applies, "ET" is the safe general term for the zone whatever the season.
The core difference
- EST — Eastern Standard Time, UTC−5. In effect from early November to mid-March.
- EDT — Eastern Daylight Time, UTC−4. In effect from mid-March to early November.
They never both apply. Writing "3pm EST" for a meeting in July is a common error — in July the zone is on EDT, an hour different.
The safe form: ET
ET means Eastern Time without committing to the season, and it is what broadcasters and most schedulers use. If you write ET, the reader converts using whatever is in force on the date. If you write EST in summer, you have specified a time an hour earlier than you probably meant.
The same pattern in the other zones
- Central — CST (UTC−6) and CDT (UTC−5)
- Mountain — MST (UTC−7) and MDT (UTC−6)
- Pacific — PST (UTC−8) and PDT (UTC−7)
Where daylight time is not observed
Most of Arizona stays on Mountain Standard Time all year, as does Hawaii on its own zone. So "MST" in Arizona in July is correct, while "MST" in Colorado in July is not — the same abbreviation is right in one state and wrong in the next.
The wider point for scheduling
Because the changeover dates differ between countries, the gap between two cities is not constant. The UK and the US shift on different weekends, so London and New York are briefly four hours apart rather than five, twice a year. Naming the zone rather than the offset avoids the trap.
Frequently asked questions
What is the difference between EST and EDT?
EST is Eastern Standard Time, UTC−5, used in winter. EDT is Eastern Daylight Time, UTC−4, used in summer. They never apply at the same time.
Which should I write for a meeting?
ET, unless you are certain of the season. ET means Eastern Time generally and lets the reader convert correctly for the date.
Does everywhere in the US change clocks?
No. Most of Arizona and all of Hawaii do not observe daylight saving, so their standard abbreviations remain correct year round.