The SQL standard is unfortunately really horrible when it comes to handling of time. The type `timestamp` is not a timestamp at all because it doesn't encode a unique point in time, it just stores a date and a time which has to be interpreted relative to a timezone. It should be called "datetime".
Moving a Java Instant back and forth between a database is also a surprisingly difficult task to do right, and it doesn't help that JDBC is just handling it completely wrong if you use its setTimestamp/getTimestamp methods. Not because it is a bad design with footguns, but because the implementation is just plain wrong and will corrupt your data if you deal with instants whose calendar date is far enough in the past due to it using the legacy date/time API which switches to the Gregorian calendar for dates in the past.
The name `timestamp with time zone` is also misleading because it doesn't actually store a time zone, it stores the number of seconds since epoch like a java.time.Instant (although at a different resolution). The "with time zone" part just refers to the textual format you denote the values in which includes the time zone after the date/time part to uniquely identify a timestamp, but the time zone is thrown away and not stored after the value has been parsed. This is different from e.g. `ZonedDateTime` in Java which will actually store the offset and therefore corresponds to a pair of (Instant, TimeZone).
A ZonedDateTime is not an instant and a timezone, for the same reason that you can’t unambiguously round trip between arbitrary timezones and UTC:
- zoned datetimes carry ambiguities as to their actual location on the timeline (because they can repeat, or not exist at all)
- future zoned datetime carry outright uncertainty as to their actual location on the timeline (as the zone’s offset can be updated at any point and any number of times until the event has elapsed)
Has anyone proposed versioning timezones? Or is this such an edge case it would be overkill? (Either specify your future instant in UTC if you mean to stick to that, or specify it in a timezone and accept that it could change before it happens, or if you need something else get it in a contract and don't trust the computer!)
tzdb is versioned, but I can't recall if the individual timezones carry a version though...
One way to manage is to store the datetimes with a timezone identifier and the offset, and when you load a new tzdb, go through and validate that the calculated offset matches the stored offset... for those events where they don't match, you have an exciting challenge of figuring out if the event should stay with the time zone or stay with the offset; both answers may be right ... ideally you inform the user(s) about what you've done and allow them to fix things software has messed up.
JS’s temporal API will serialize zoned datetimes with both offset and timezone (specifically, an RFC 9557 representation), this can be used to infer tzdb updates. By default, temporal will raise an error when parsing a datetime where the offset and the timezone’s offset differ, but it can be configured to sway one way or the other depending on use case.
ulrikrasmussen · · focus · HN ↗
Moving a Java Instant back and forth between a database is also a surprisingly difficult task to do right, and it doesn't help that JDBC is just handling it completely wrong if you use its setTimestamp/getTimestamp methods. Not because it is a bad design with footguns, but because the implementation is just plain wrong and will corrupt your data if you deal with instants whose calendar date is far enough in the past due to it using the legacy date/time API which switches to the Gregorian calendar for dates in the past.
The name `timestamp with time zone` is also misleading because it doesn't actually store a time zone, it stores the number of seconds since epoch like a java.time.Instant (although at a different resolution). The "with time zone" part just refers to the textual format you denote the values in which includes the time zone after the date/time part to uniquely identify a timestamp, but the time zone is thrown away and not stored after the value has been parsed. This is different from e.g. `ZonedDateTime` in Java which will actually store the offset and therefore corresponds to a pair of (Instant, TimeZone).
masklinn · · focus · HN ↗
- zoned datetimes carry ambiguities as to their actual location on the timeline (because they can repeat, or not exist at all)
- future zoned datetime carry outright uncertainty as to their actual location on the timeline (as the zone’s offset can be updated at any point and any number of times until the event has elapsed)
rawling · · focus · HN ↗
toast0 · · focus · HN ↗
One way to manage is to store the datetimes with a timezone identifier and the offset, and when you load a new tzdb, go through and validate that the calculated offset matches the stored offset... for those events where they don't match, you have an exciting challenge of figuring out if the event should stay with the time zone or stay with the offset; both answers may be right ... ideally you inform the user(s) about what you've done and allow them to fix things software has messed up.
masklinn · · focus · HN ↗