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And the "changes to past timestamps" is why you can't even store past events using Olson timezone names like America/Los_Angeles: you have to use UTC and/or tim

And the "changes to past timestamps" is why you can't even store past events using Olson timezone names like America/Los_Angeles: you have to use UTC and/or timezone offsets like -0700. https://en.m.wikipedia.org/wiki/Tz_database https://en.m.wikipedia.org/wiki/Tz_database https://news.ycombinator.com/item?id=10032498 https://news.ycombinator.com/item?id=10032498

0x0 8y ago
Is it not the opposite, though? If you need to store the fact that an important meeting took place in Los Angeles at exactly 4.00pm on a specific date, if you had used utc/offsets, it would look like the meeting took place at 3.59.50pm or whatever change was done to correct the tz data for historical timezones?
- MattHeard 8y ago
  A timestamp should not be used to record location.
- treve 8y ago
  It really depends on the use-case. If you had a meeting at a specific wall-time you'll want the timestamp to change. If you recorded a timestamp for when something happened, the local time should change. But you should under no circumstances record the offset. Record either a timestamp or local time + location.
  - mikelward 8y ago
    I agree that systems should record times in UTC. But I'm talking about a more nuanced edge case: parsing user-entered date strings. If a user says "4pm Los Angeles", it could be ambiguous, and recording the offset is the only way to reliably disambiguate. See my reply above.
- mikelward 8y ago
  As an extreme example, imagine everyone thought daylight saving was going to happen on March 11. But at the last minute, the government announces it's happening on March 4. And your systems haven't yet been updated to reflect this. So humans move their clocks forward one hour on March 4. You write somewhere in a text field "March 4 4pm". "March 4 4pm America/Los_Angeles" doesn't help you, because it's not clear whether you thought "America/Los_Angeles" was -0800 or -0700. And it's not clear whether the system interpreting that date agrees with you. To be unambiguous, you need to say "March 4 4pm -0700". Otherwise the system might assume the effective timezone is -0800, per the rules it knows. Without unambiguous input, correctly parsing such times is impossible. You could also use Olson timezones for past events, but then you'd also have to record which version of the timezone rules you were using. And of course nobody does that because it's overcomplicated and unnecessary. You're right that converting back to 4pm for display is problematic. The simplest solution is to always display UTC. But if you want to convert to the user's preferred timezone, during this time, yes, the system would display 3pm until it got the updated timezone rules. There are more complex solutions, but for most use cases just displaying "March 4 3pm -0800" would be adequate.

0x0 8y ago

Is it not the opposite, though? If you need to store the fact that an important meeting took place in Los Angeles at exactly 4.00pm on a specific date, if you had used utc/offsets, it would look like the meeting took place at 3.59.50pm or whatever change was done to correct the tz data for historical timezones?

MattHeard 8y ago
A timestamp should not be used to record location.
treve 8y ago
It really depends on the use-case. If you had a meeting at a specific wall-time you'll want the timestamp to change. If you recorded a timestamp for when something happened, the local time should change. But you should under no circumstances record the offset. Record either a timestamp or local time + location.
- mikelward 8y ago
  I agree that systems should record times in UTC. But I'm talking about a more nuanced edge case: parsing user-entered date strings. If a user says "4pm Los Angeles", it could be ambiguous, and recording the offset is the only way to reliably disambiguate. See my reply above.
mikelward 8y ago
As an extreme example, imagine everyone thought daylight saving was going to happen on March 11. But at the last minute, the government announces it's happening on March 4. And your systems haven't yet been updated to reflect this. So humans move their clocks forward one hour on March 4. You write somewhere in a text field "March 4 4pm". "March 4 4pm America/Los_Angeles" doesn't help you, because it's not clear whether you thought "America/Los_Angeles" was -0800 or -0700. And it's not clear whether the system interpreting that date agrees with you. To be unambiguous, you need to say "March 4 4pm -0700". Otherwise the system might assume the effective timezone is -0800, per the rules it knows. Without unambiguous input, correctly parsing such times is impossible. You could also use Olson timezones for past events, but then you'd also have to record which version of the timezone rules you were using. And of course nobody does that because it's overcomplicated and unnecessary. You're right that converting back to 4pm for display is problematic. The simplest solution is to always display UTC. But if you want to convert to the user's preferred timezone, during this time, yes, the system would display 3pm until it got the updated timezone rules. There are more complex solutions, but for most use cases just displaying "March 4 3pm -0800" would be adequate.