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What is the difference between PT (Pacific Time), PDT (Pacific Daylight Time), and PST (Pacific Standard Time)? Also, is the time format "2:00pm PT" unambiguous?

Mateen Ulhaq
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Sandro
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    Please try to avoid these abbreviations if any part of your audience lives outside of North America. –  Oct 30 '12 at 06:52
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    Pacific Time is either Pacific Daylight Time or Pacific Standard Time. In some contexts Pacific Standard Time may be interpreted as including the possibility of a Daylight Time variant, in other contexts not. Pacific Daylight Time is always "summer time" and an hour different from the "winter" version of Pacific Standard Time. – Hot Licks Apr 02 '15 at 22:56
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    Normally it's sufficient to avoid the whole issue and say "I'll meet you at 2:00pm, San Francisco time (or 'local time')." For a given location there is no ambiguity in the "local time". – Hot Licks Apr 02 '15 at 23:01
  • @HotLicks Unless that location is Tuba City, AZ or College Corner, OH/IN... – Dan Henderson Apr 07 '17 at 14:31
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    @DanHenderson - In those cases you're better off making your date in UTC terms. – Hot Licks Apr 07 '17 at 19:20
  • In a particular country, no one uses UTC. That is used in international communications to avoid misunderstandings. – Lambie Nov 16 '21 at 15:23

4 Answers4

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My understanding, and the way things are typically named at least in the context of computing (I'm a programmer by trade), has always been the following:

  • PDT = Pacific Daylight Time = fixed to UTC-7
  • PST = Pacific Standard Time = fixed to UTC-8
  • PT = Pacific Time = a general reference to the time zone, which alternates between PDT and PST depending on the time of year.

Colloquially, people seem to like using PST and PT interchangeably, and will still (IMO, incorrectly) refer to times as PST even when daylight savings time is in effect and Pacific Time is UTC-7.

Also, colloquially, I have seen PDST sometimes used interchangeably with PT to refer to the timezone that alternates between PDT and PST.

This is all oversimplified, as daylight savings rules are different in different cities and countries. With that in mind, another way of viewing it is:

  • PDT: A fixed, standard time, always UTC-7.
  • PST: A fixed, standard time, always UTC-8.
  • PT (or rarely, PDST): A vaguer concept taken to mean "whatever UTC offset is currently in effect in the Pacific Timezone, in the location given by context".

Usually, then, when talking to humans (as opposed to computers) you'd use PT (or, colloquially, PST, even though it's arguably incorrect) to mean "whatever time it is in Pacific Time in whatever location we're talking about".

So:

"I'll meet you in San Francisco Nov 1st, 5:00 PM PT" means "on November 1st, when clocks in San Francisco say 5:00 PM", which in that case would specifically be 5:00 PM PDT (UTC-7).

Likewise, "I'll meet you in San Francisco Nov 3rd, 5:00 PM PT" means "on November 3rd, when clocks in San Francisco say 5:00 PM", which in that case would be 5:00 PM PST (UTC-8).

Mountain Time, for example, is slightly more complicated than Pacific Time because it covers a larger area with more diverse daylight savings rules (e.g. much of Arizona does not observe daylight savings time).

In computing we tend to just represent times in UTC to avoid ambiguity.

Talking to a human you'll be fine saying "Pacific Time", "PT", or "PST" (which, again, has come to be interchangeable with "PT" although purists like myself believe otherwise :) ). If somebody says "PST" to you, 99% of the time they just mean "Pacific Time" in general as per current daylight savings time rules.

I believe the use of "PST" to mean "Pacific Time" has become so common in language that it's pretty much "official".

Jason C
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  • Can you cite some sources. Especially for the assertion that "If somebody says 'PST' to you, 99% of the time they just mean "Pacific Time" in general as per current daylight savings time rules." – orome Apr 02 '15 at 13:56
  • @rax Sure thing, I will see what I can dig up. It will have to wait a few days, I am on a job right now. – Jason C Apr 02 '15 at 15:14
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    Wanted to add some anecdata: as a native speaker of American English in California, this description of how PST is used colloquially seems spot on. – AlannaRose Jul 16 '15 at 23:18
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    I'm an example. I live in California and all year round I personally have been using PST exclusively when referring to the time in California. :) – EnabledZombie Mar 09 '16 at 22:03
  • I had a meeting with someone from San Francisco and it was scheduled to PST. Originally I thought it was Standard Time, but even Google Calendar converted it to PDT. So I went on to confirm and the person told me that PST and PDT are the same and Google normally converts so it avoids this Standard Time confusion. Summing up, so probably PST is PDT which is one hour ahead of PT. – lucasarruda Oct 21 '16 at 16:37
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    This gives me "PTSD". – supyo Jul 06 '18 at 20:37
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    The sort of confusion described in this answer is typical. Most people do not grasp that the three-letter acronyms like PST and AST and IST and so on describe UTC offsets, not geographical timezones. As a Brit, I regularly encounter people who are surprised to learn that the UK is not on GMT the whole year round. – Mark Amery Dec 06 '18 at 16:30
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    Let's just all admit we have so much trouble tracking PST vs. PDT that we say "PT" and hope the other person will assume "whatever daylight savings phase we're in". – jimm101 Nov 16 '21 at 17:33
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To avoid ambiguity or confusion it's best to follow established practice when specifying a time. "PT" refers not to a time, but to a time zone, and can therefore be used (implicitly and unambiguously) to refer to the current time in the Pacific time zone.

If you specify a time (rather than a time zone), you should use the correct one (PDT or PST). Using PST to mean "the current time in the Pacific zone" is wrong (since it not only conflates a time zone with a time, but also implicitly makes "PDT" and "PST" synonyms for nearly half of the year).

orome
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Per Time and Date's table of Time Zone Abbreviations for North America:

  • An "other name" for Pacific Standard Time and Pacific Daylight Time is "PT" (instead of the more specific "PST/PDT");
  • And an "other name" for Eastern Standard Time and Eastern Daylight Time is "ET" (instead of the more specific "EST/EDT").
t_l
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Kristina Lopez
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    +1 The Abbreviation chart at http://www.timeanddate.com/library/abbreviations/timezones/ is frightening... CST can mean China Standard Time (UTC+8), Central Standard Time (UTC-6), or Cuba Standard Time (UTC-5). – rajah9 Oct 30 '13 at 19:34
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In most cases, PT is specific enough. The only exceptions would be on the day the clocks change. Also, in common speech, people often use PST when they mean PT.

Charles
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    On any given day, many folks will have trouble remembering if they are on PST or PDT. You can get around that by saying PT, which means 2PM in the Pacific time zone, irrespective of whether or not the zone is in Daylight or Standard time. So, I agree with Charles - not only does PT suffice, it might also be less confusing. – J.R. Oct 29 '12 at 19:07
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    I always forget that as well, which is why I tend to use PT. I'd rather write PT instead of getting PDT and PST mixed up. – Sandro Oct 29 '12 at 19:27