The short answer

EST is UTC−5. EDT is UTC−4. They are one hour apart and only ever one of them is in effect.

New York uses EST from early November to mid-March, and EDT from mid-March to early November — so for about eight months of the year, writing “3pm EST” describes a time that does not exist anywhere. If you mean “whatever New York is on right now,” the correct term is Eastern Time (ET).

The two names are a claim about the date

Most people use “EST” as a name for a place. It is not. It is a name for a place in a particular half of the year.

Term	Offset	In effect	What it actually means
EST	UTC−5	Nov – Mar	Eastern Standard Time — the winter clock
EDT	UTC−4	Mar – Nov	Eastern Daylight Time — the summer clock
ET	either	always	Eastern Time — “whichever of the two applies today”

This is verifiable rather than a matter of style. Right now the Eastern zone (America/New_York in the IANA database) reports EST / UTC−5 in January and EDT / UTC−4 in July.

Why this specifically wrecks meetings

The failure is not that someone mistypes an offset. It is that “EST” looks precise, so nobody questions it.

Three ways it goes wrong:

Summer invitations written in winter’s vocabulary. A calendar entry says “4pm EST” in June. Half the attendees read it as “4pm in New York” (which is EDT, UTC−4). The other half — or a scheduling tool — reads it literally as UTC−5 and arrives an hour late.
Non-US attendees taking it literally. Someone in London converting from a stated UTC−5 in July lands an hour off, and they were right to trust what was written.
Recurring calls across the transition. The invite does not change. The offset underneath it does, twice a year, and for a few weeks the US and Europe are not even changing on the same dates. That gap has its own page.

What to write instead

Best: the city. “3pm New York.” Unambiguous in every month, and it is what the IANA database is keyed on anyway.
Good: “3pm ET.” Correct year-round because it does not claim an offset.
Fine when the date is fixed and you have checked it: “3pm EDT, 14 July.”
Avoid: “EST” unless you genuinely mean the winter offset, on a winter date.

A useful habit: if you are writing an offset for a date more than a couple of weeks out, write the city instead. The offset can change between now and then. The city cannot.

The same trap, in every other US zone

Eastern is the one people get wrong most often, but the pattern is identical across the country:

Zone	Standard (winter)	Daylight (summer)
Eastern	EST · UTC−5	EDT · UTC−4
Central	CST · UTC−6	CDT · UTC−5
Pacific	PST · UTC−8	PDT · UTC−7
Arizona	MST · UTC−7	MST · UTC−7

Arizona is the instructive one. It stays on MST all year, so its gap to New York is 2 hours ahead in winter and 3 hours ahead in summer — the same two places, a different answer depending on the month, without Arizona doing anything at all.

The one case where EST never changes

Some places that use UTC−5 do not observe daylight saving, so their offset is genuinely fixed year-round even though the US Eastern zone’s is not. That is why converters keyed on a city stay correct while converters keyed on an abbreviation drift.

It is also why abbreviations are a poor identifier in general: several are ambiguous across countries, and the IANA database deliberately uses Area/City names instead.

What to remember

EST and EDT are one hour apart and mutually exclusive.
“EST” in July is almost certainly meant as ET, and is technically wrong.
Write the city, not the offset, whenever the date is not immediately next.
Two places can both “not change their clocks” and still drift apart, because the other one does.

Related: What is EST? · PST to EST · IST to EST

Timezone: World Clock Widget

A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.

What it does

Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.

EST vs EDT: the difference that ruins meetings