Falsehoods Programmers Believe About Time: the classic list every engineer should reread
aronchick · x · 2026-10-09
Noah Sussman's classic essay Falsehoods Programmers Believe About Time keeps resurfacing: after years debugging others' test code, the author found countless bugs stem from wrong assumptions about time and calendars.
The list of always-wrong assumptions includes:
- There are always 24 hours in a day
- Months have either 30 or 31 days
- Years have 365 days; February is always 28 days
- Any 24-hour period begins and ends in the same day/week/month
- The machine always runs in GMT
- The production time zone will never change
- The system clock is always correct
Daylight saving is just the tip of the iceberg — timezone rule changes and clock drift break all of these assumptions.
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