Standard and Daylight are different

A couple weeks ago I had an online meeting with some European colleagues; I showed up in the chat room at what I thought was the agreed-upon time and they did not, which was odd, but whatever, I waited ten minutes and then rescheduled the meeting. It turns out they did the same an hour later. I’m sure you can guess why.

If you have been sent a link to this page, it is to remind you that “Eastern Standard Time” is not defined as “whatever time it is in New York City right now”, it is defined as “Eastern Time not adjusted for Daylight Saving Time“. Parts of the world in the eastern time zone that do not observe Daylight Saving Time — Panama, for instance — stay in Eastern Standard Time all year, so it is an error to assume that Eastern Standard Time and Eastern Time are the same time.

Put another way: Universal Time Coordinated (UTC) is the time in Greenwich not adjusted for British Summer Time, which is what the Brits sensibly call Daylight Saving Time. Eastern Standard Time is always UTC minus five hours, and Eastern Daylight Time is always UTC minus four hours. Eastern Time switches between EST and EDT depending on the time of year.

So when you tell someone that the meeting is at noon on Eastern Standard Time on a day in April, you are saying that the meeting is at noon Jamaican time, not noon New York City time. Since computers do what you tell them, not what you mean, you might find that setting an online intercontinental meeting for a time in “EST” might give you a different time than you think. Your best bet is to state the time in UTC, which is unambiguous.

23 thoughts on “Standard and Daylight are different

  1. I’ve seen my fellow Britons state a time suffixing “Greenwich Mean Time” for patriotic emphasis. I’ve often had to ask “Do you actually mean Greenwich Mean Time as we’re in the summer time zone right now?”

    • I used to work on airline software in Ireland, which is in the same time zone.

      As you would expect for pilots, all displayed times including the system clock of the device the software was running on was always in Zulu time (military name for UTC).

      As soon as daylight saving started, I got a bug report – all the times were one hour out. The software didn’t adjust for it. Ha ha. Easy fix.

  2. That’s why for anything online and cross-timezone one better use UTC. When will people learn that local values require marshalling?

  3. I always try to give the time in their timezone and do the adjustment on my end following the UX advice, “don’t make [them] think.”

  4. I had friends in Arizona (one of the last holdouts in not observing daylight saving) who complained about people who misuse the term.

    They would sometimes call companies in other states and get a recording saying their business hours are “X am to Y pm Mountain Standard Time”. Since Arizona is always in Mountain Standard Time, you’d expect not to have to convert, but most of the time the people making those recordings meant “Moutain Time” or “Mountain Prevailing Time”. The Arizonans have to figure out what state the call center is in and whether it’s currently DST in the rest of the US in order to figure out the actual business hours.

  5. That’s how you learn that most of us coming from Europe are not accustomed with US names for timezones and are used with GMT+/-n notation (UTC is rarely used, but it’s assumed to mean the same time).
    Even the local names for the European timezones (CET, EET etc.) are rarely used, and people prefer to use GMT+1, GMT+2 etc.
    So for us it would be easier to think in terms of UTC-5 than EST.. 🙂

  6. When corresponding with non-US people, I always refer to the time in a particular city. “Let’s do this at 10 a.m., New York time/3 p.m. London time,” for example. This implicitly incorporates each location’s civil rules concerning the adjustment of clocks.

    Time differences are particularly troubling when locations are in jurisdictions that change their clocks on different days. This means there is a short period when the time difference is less or greater than usual. Saying the name of the city gives an opportunity for the correspondents to mention the date on which the clocks will change. Still, that requires them to think about whether the clocks will change, so the UTC solution is less ambiguous still. I don’t say it’s better, however, because many of the people I set meetings with in Europe would probably have no idea what I was talking about if I scheduled a meeting for 14:00 UTC.

    When I applied to graduate school in Bloomington Indiana, I received a letter from the department secretary that mentioned that “Bloomington is on Eastern Standard time.” I was living in Connecticut at the time. After the clocks changed to daylight saving time, I missed a phone appointment because I assumed that clocks had changed in Bloomington, too. I asked the secretary about this, and she said, “well, it does say we’re on Eastern Standard Time.”

    I was incredulous. True, it was literally correct, but given the widespread misuse of “xyz standard time” to mean “the time in the xyz zone” (not to mention the fact that nobody would ever say “my town is on xyz standard time sometimes and xyz daylight time the rest of the time”) I always thought it would make more sense to be somewhat more explicit about the fact that Bloomington did not observe daylight saving time.

  7. In Australia, we also have 3 time zones (Eastern, Central, and Western), and those states and territories that lie mostly, or entirely in the Tropics (Queensland, Northern Territory) don’t use the Daylight Saving time (while the other ones do). This creates a bit of mess with, say, Eastern Standard Time (EST, Brisbane, Cairns) or Eastern Daylight Time (EDT, Sydney, Melbourne). For half-a-year they are the same, for another half they aren’t. Plus, it’s winter in the US and UK when it’s summer here. So anyone scheduling an online meeting with the down-under-ish folks, beware!. 🙂

  8. To me the question is why was EST involved in the first place? Supposedly neither you nor the Europeans should be anywhere near it.
    Perhaps I should ask Jon, he seems to know everything about time, or has he already gone Zen and now defies all concepts of time because they are not properly rooted?

  9. We have teams and customers around the world, we always use UTC and put UTC in emails. Our clients (in Scandanavia and the Far East) refer to it as GMT. Which is odd, if you think about it – Brits using “UTC” as a moniker and everyone else using “GMT”. I think it’s us trying really hard not to be Imperial any more!

  10. The solution to the whole issue of time zones is to get rid of them entirely, and collectively switch to UTC. Sure, “8 PM” would stop having the meaning of being in the beginning of the evening, but really, why would that be a problem?

    I fear it might require some brisk convincing here and there. I think starting with Putin is my best bet. So he can demonstrate he is not also the boss of Russia, but also the boss of time.

    • Before such a massive convincing feat can be accomplished, I think we’ll all be using smart watches with GPS that will auto-adjust to the local time wherever we are — and show us the current time at any place we ask. 🙂

  11. Minor interesting point. GMT and UTC are not quite the same. GMT, like all other timezones, is defined as a shift from UTC (in this case +0). UTC is time defined by atomic clocks and is responsible for handling leap seconds/years.

    It’s taking separation of concerns and applying it to time, allowing a timezone to be a shift in local clocks from a unified definition of time. Basically GMT == UTC but GMT !==UTC.

  12. If both parties schedule their meeting via Outlook, they just need to wait for the 15-minute reminder. I don’t recall being invited to a meeting without it. If the notification is missing, one is probably early; if it shows up unexpectedly, one is likely to be (almost) late.

  13. I always use UTC and really hate when something glabally available is announce to happen in EST, EDT, PDT and all those acronims.

    Let’s be serious and use just UTC for those things on use this besides your local time. It’s easier for foreigners.

  14. There are a couple of other good ways of doing it. Just put your local time (ie when you want to have the meeting at your end) into your scheduling tool (ie calendar) and then let it work out what timezone that is for anybody else invited. Or if you are going to put stuff in somebody else’s timezone put “I think” and let them confirm your calculations. 😉

  15. “Universal Time Coordinated (UTC) is the time in Greenwich not adjusted for British Summer Time”

    Well, that’s true most of the time. It wasn’t true between October 1968 and October 1971, when Greenwich observed BST – not British Summer Time, but British Standard Time, which was still UTC+1, but where the +1 was a *standard* offset rather than a daylight saving offset.

    (For more trivia like this, see

    Of course it doesn’t help that Windows (e.g. TimeZoneInfo) uses the “standard time” name as the ID…

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