Notes on Date and Time

Backend (server, database)

always store date and time in ISO 8601 format, UTC timezone (e.g. 2023-01-01T00:00:00Z)
when necessary to store the timezone, store it separately from the date and time
always use libraries for date and time manipulation (e.g. date-fns) ¹
API endpoints should accept either ISO datestrings or timestamps ²
JS uses milliseconds for timestamps, other languages (e.g. PHP) use seconds ³

Frontends

see ¹
see ²
see ³

Date object

toLocaleDateString et al.: always explicitely pass formatting options to avoid unexpected results in some locales
only create dates from string when ISO
it's useful to store user-provided date and time values in separate variables as numbers (e.g. new Date(year, zeroIndexedMonth, day, hours, minutes, seconds))

Formats

never take the order for granted (e.g. 11/10/1990 could mean 1990-10-11 or 1990-11-10)

Node.js

make sure full-icu is enabled when running test suites or formatting might default to en-US