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randommoniclerandommonicle/claude-skills/date-parse-utc-safe/SKILL.md

date-parse-utc-safe

Skill27 starsChanged 18 days agoApache-2.0View on GitHub

In a UTC server or serverless runtime, new Date("YYYY-MM-DD") is UTC midnight and renders as the previous day in western timezones. Parse date-only strings into explicit local components (new Date(y, m-1, d)) anywhere a date becomes a display string or a day-of-week. Triggers when parsing a date-only string server-side or computing a printed day from one. Does not fire on full ISO timestamps carrying explicit timezones.

The file

We split this skill into the rules it gives an agent

2 rules. About 0 are common, 1 is distinctive, and 1 is very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈1

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 1

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Very specific:

    Never construct a Date from a bare YYYY-MM-DD string for display or day-of-week.

    line 15
  2. Distinctive:

    Test the boundary: a date rendered at local midnight in the runtime's timezone is where the off-by-one appears.

    line 23

≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

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.

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