What is enough to store dates/times in the DB from multiple time zones for accurate calculations?

Question

This is a HARD question. In fact it is so hard it seems the SQL standard and most of the major databases out there don't have a clue in their implementation.

Converting all datetimes to UTC allows for easy comparison between records but throws away the timezone information, which means you can't do calculations with them (e.g. add 8 months to a stored datetime) nor retrieve them in the time zone they were stored in. So the naive approach is out.

Storing the timezone offset from UTC in addition to the timestamp (e.g. timestamp with time zone in postgres) would seem to be enough, but different timezones can have the same offset at one point in the year and a different one 6 months later due to DST. For example you could have New York and Chile both at UTC-4 now (August) but after the 4th of November New York will be UTC-5 and Chile (after the 2nd of September) will be UTC-3. So storing just the offset will not allow you to do accurate calculations either. Like the above naive approach it also discards information.

What if you store the timezone identifier (e.g. America/Santiago) with the timestamp instead? This would allow you to distinguish between a Chilean datetime and a New York datetime. But this still isn't enough. If you are storing an expiration date, say midnight 6 months into the future, and the DST rules change (as unfortunately politicians like to do) then your timestamp will be wrong and expiration could happen at 11 pm or 1 am instead. Which might or might not be a big deal to your application. So using a timestamp also discards information.

It seems that to truly be accurate you need to store the local datetime (e.g. using a non timezone aware timestamp type) with the timezone identifier. To support faster comparisons you could cache the utc version of it until the timezone db you use is updated, and then update the cached value if it has changed. So that would be 2 naive timestamp types plus a timezone identifier and some kind of external cron job that checks if the timezone db has changed and runs the appropriate update queries for the cached timestamp.

Is that an accurate solution? Or am I still missing something? Could it be done better?

I'm interested in solutions for MySQL, SQL Server, Oracle, PostgreSQL and other DBMS that handle TIMESTAMP WITH TIME ZONE.

Answer 1

You've summarized the problem well. Sadly the answer is to do what you've described.

The correct format to use does depend the pragmatics of what the timestamp is supposed to represent. It can in general be divided between past and future events (though there are exceptions):

• Past events can and usually should be stored as something which can never be reinterpreted differently. (eg: a UTC time stamp with a numeric time zone). If the named time zone should be kept (to be informative to the user) then this should be separate.

• Future events need the solution you've described. Local timestamp and named time zone. This is because you want to change the "actual" (UTC) time of that event when the time zone rules change.

I would question if time zone conversion is such an overhead? It's usually pretty quick. I'd only go through the pain of caching if you are seeing a really significant performance hit. There are (as you pointed out) some big operations which will require caching (such as sorting billions of rows based on the actual (UTC) time.

If you require future events to be cached in UTC for performance reasons then yes, you need to put a process in place to update the cached values. Depending of the type of DB it is possible that this could be done by the sysadmins as TZ rules change rarely.