Proper Timezone Acronym Usage - PT vs PDT or PST

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".

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).

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").

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.