Am 17.09.2018 um 21:29 schrieb Neil Mayhew:
On 2018-09-17 12:01 AM, Tobias Dammers wrote:
… you can convert unix timestamps to some "local time without timezone info" data structure (e.g. LocalTime); whether you assume UTC or any timezone becomes relevant when you want to convert that data structure into a data structure that does contain timezone info explicitly (like ZonedTime) or implicitly (like UTCTime).
I don't think this is possible. If you want to measure seconds since 1970-01-01 00:00:00 local time (which may actually be 1969 in UTC) you have to know the timezone at the start (ie in 1970) and at the end (ie after the seconds have elapsed). That requires knowing whether DST applies at either end, and whether the location was rezoned during the intervening time.
Actually you need only know the time offset at start and end of interval. Any intervening changes cancel out. You do need to know whether the location had its time zone changed (which happened multiple times in some areas I believe).
This requires IO and the conversion from seconds is no longer a pure operation.
Eek. IO just to handle timezone is a really bad idea. Just use a timestamp-with-timezone data type for the parameters and you're back with pure functions.
IHMO, the only sane way to interpret seconds-since-the-epoch is using UTC.
Now that's true. From the point of view in the paragraph above, simply because it's lacking the data needed for any other interpretation. Note that there are excellent designs for time representation arounds. A good one would have to cover instants (multiple representations), intervals (with and without leap seconds), time zones, and calendars. From my daily Java work, the one that works best is the Jodatime library, documented at http://www.joda.org/joda-time/. If anybody wishes to check whether it has good ideas worth stealing, take a look at the Javadoc at http://www.joda.org/joda-time/apidocs/index.html. If you want to be really thorough, you can also study JSR-310, which was supposed to be a slightly improve version of Jodatime, AND part of the Java standard library. That work stopped when it was "good enough", which isn't good enough. Still, the differences between Jodatime and JSR-310 may be interesting in themselves (they may show areas where the Jodatime author believed his work could be improved, as he was deeply involved in the JSR as well). Anyway: the JSR-310 classes are documented at https://docs.oracle.com/javase/8/docs/api/index.html, in the java.time package and subpackages. Regards, Jo