Formatting and correctness.
WHAT GOES WRONG WITH DATES
Time zones Formats differing by locale Ambiguity between day and month Arithmetic across daylight changes
WHAT TO STORE AND TRANSMIT
A standard unambiguous format, in a single reference time zone.
WHAT TO DISPLAY
The user's local time, formatted for their locale.
WHAT TO USE
The platform's internationalisation facilities, or an established library.
WHAT NOT TO DO
Format dates by joining strings Assume the user's time zone matches the server's
WHAT TO BE CAREFUL WITH
Dates without times, which should not be converted between zones.
WHY
A birth date shifting by a day is a common and confusing bug.
WHAT TO NEVER USE FOR MONEY
Floating point arithmetic.
WHY
It produces rounding errors, visibly.
WHAT TO USE INSTEAD
Integer minor units, or a decimal library.
WHAT TO FORMAT WITH
The platform's number formatting, which handles separators and currency symbols by locale.
WHAT TO DECIDE ONCE
Where rounding happens, and apply it consistently.
