Let me explain the difference between relative vs absolute a bit more.Ībsolute time is used to record an event. Random programming language note: Python's datetime data type is very good at maintaining the distinction between absolute vs relative times (albeit frustrating at first until you supplement it with a library like PyTZ). also use UTC as the timezone in the database if possible.never mix and match relative and absolute timestamps.treat all non-zero UTC times as an input error.use UTC for everything unless there is a compelling reason to store a non-zero offset.America/Los_Angeles) and not an offset from UTC (e.g. store a user’s time zone as a named label (e.g. Creates a relative time in a given frame of reference (i.e. Create an absolute time with timezone offset: Test=> SELECT EXTRACT(TIMEZONE_HOUR FROM TIMESTAMP WITH TIME ZONE ' 22:48:02.235541') Test=> SELECT EXTRACT(TIMEZONE_HOUR FROM NOW()) When converting from an incomplete TIMESTAMP WITHOUT TIME ZONE to a TIMESTAMP WITH TIME ZONE, the missing time zone is inherited from your connection: test=> SET timezone = 'America/Los_Angeles' Note that AT TIME ZONE 'UTC' strips time zone info and creates a relative TIMESTAMP WITHOUT TIME ZONE using your target’s frame of reference ( UTC). For example, one such feature: - Make sure we're all working off of the same local time zone There are lots of opinions on how to do this, but this seems to be the best in practice from my experience.Ĭriticisms of database time zone handling is largely justified (there are plenty of databases that handle this with great incompetence), however PostgreSQL’s handling of timestamps and timezones is pretty awesome (despite a few "features" here and there). It's not 100% perfect, but it provides a strong enough anti-footshooting measure that makes sure the data is already converted to UTC. Test=> INSERT INTO my_tbl (my_timestamp) VALUES (NOW()) ĮRROR: new row for relation "my_tbl" violates check constraint "my_tbl_my_timestamp_check" Test=> SET timezone = 'America/Los_Angeles' My_timestamp TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),ĬHECK(EXTRACT(TIMEZONE FROM my_timestamp) = '0') To ensure data going in to the database is stored with an offset of zero: CREATE TABLE my_tbl ( PostgreSQL is very adept at converting times between any two arbitrary time zones). Again, PostgreSQL doesn’t need this to do date calculations correctly or to convert between time zones (i.e. a script dumps data to a file and something else sorts the time data using a lexical sort). Setting a CHECK on the column to make sure the write to the time stamp column has a time zone offset of 0 is a defensive position that prevents a few classes of bugs (e.g. This isn’t a PostgreSQL requirement, but it helps when integrating with other programming languages or environments. Don't ever, ever, ever mix absolute and relative TIMESTAMPs.įrom a programmatic and consistency perspective, ensure all calculations are made using UTC as the time zone. WITHOUT TIME ZONE yields a relative time zone. Like Option 3, the reason for the WITH TIME ZONE is because the time at which something happened is an absolute moment in time. That said, let me explain some background aspects of this this Option 4 in more detail. This option doesn’t always work because it can be hard to get a user’s time zone and hence the hedge advice to use TIMESTAMP WITH TIME ZONE for lightweight applications. convert from UTC to America/Los_Angeles).
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