Eye Training · Time and Dates

A Date is one instant. Every bug comes from forgetting which calendar, which time zone or which unit you're reading it in.

Part 2 · Surface 23. Bug counts are hidden and some snippets are clean. See Start Here for how to use these drills.

J · The basics

M · Three years in, these should jump out

S · Where seniors stand out

What your eye checks

Whenever a snippet builds, parses, compares or stores a date, ask:

1. Which index? Months start at 0. getDay is the weekday and getDate the day of the month.
2. Was the parse valid? An Invalid Date is truthy. Check getTime() for NaN.
3. Which zone? A date-only string is UTC. A date-time without an offset is local. toISOString() is always UTC.
4. Which unit? JWT exp, Unix timestamps and many APIs use seconds, while JavaScript uses milliseconds.
5. Whose "today"? A day boundary belongs to a user's time zone, not the server's.
6. Is the Date shared? The set… methods change it in place.
7. What happens at the edges? The 31st, the last day of a month, a leap year, the night the clocks change.
8. Is the range half-open? >= start and < next start never drops the last day.

Tells

Go deeper

• Values, equality and numbers: NaN checks and unsafe numbers
• Previous surface: Functions, this and closures · Next: Resources and cleanup