randommonicle Date parsing, UTC-safe
One rule, endlessly rediscovered: new Date("YYYY-MM-DD") parses as UTC midnight. In a UTC
runtime (serverless platforms run UTC) rendered through a western timezone, that is the
previous day — bookings display on the wrong date, day-of-week computes wrong, and the bug
never reproduces on a developer machine in the same timezone as the users.
The rule
Never construct a Date from a bare YYYY-MM-DD string for display or day-of-week.
Decompose to explicit local components:
const [y, m, d] = dateStr.split('-').map(Number);
const date = new Date(y, m - 1, d); // local midnight, not UTC midnight
Test the boundary: a date rendered at local midnight in the runtime's timezone is where the off-by-one appears.
Why
The failure only exists in the deployed runtime's timezone, so it survives local testing by construction. Evidence: ICC LESSONS_LEARNED L-005.