A persistent source of bugs.
WHAT TO STORE
Timestamps in universal time, always.
WHY
Local times are ambiguous, and change with daylight rules.
WHAT TO STORE ALONGSIDE, WHERE RELEVANT
The time zone the event related to.
WHY
Converting for display requires it, and it cannot be recovered afterwards.
WHAT TO BE CAREFUL WITH
The server's time zone differing between environments Daylight transitions, where an hour repeats or does not exist Assuming a day is always the same length
WHAT THAT AFFECTS
Scheduled jobs running twice or not at all, during transitions.
WHAT TO USE
A tested date library, not manual arithmetic.
WHAT NOT TO DO
Add a fixed number of hours to represent a day Compare dates as text Assume the client's clock is correct
WHAT TO VALIDATE
Dates received from clients, including implausible ones.
WHAT TO SET FOR SCHEDULED WORK
An explicit time zone.
WHAT TO PREVENT
Overlapping runs of a long-running scheduled job.
WHAT TO LOG
When scheduled work ran, and how long it took.
