Find the week number for any date
ISO 8601 (Monday-start, first-Thursday rule) and US (Sunday-start) week numbers, plus day-of-year and weekday name.
How to find the week number
ISO 8601 vs US numbering disagree on which week is #1. This tool computes both — plus day-of-year and weekday — for any calendar date.
- 01
Pick a date
Use the date picker. Defaults to today.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
inputISO 8601RFC 3339epochlivedate · timestampiso · ISO2026-05-21
loaded · ISO 8601zone · ready - 02
ISO 8601 week
Monday-start. Week 1 = week containing the first Thursday.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via
Intl.DateTimeFormat.isoUTC mathIntlDST-safeliveiso · optionsW21 · live2026-W21
options applied · UTCDST-safe - 03
US week
Sunday-start. Week 1 = week containing Jan 1.
Output respects the chosen timezone offset (
Zvs+05:30) and pads digits per ISO conventions so sorts stay chronological.usoffsetzone-awaresortablepaddedus · computed21 · readyweek 21
computed · instantsortable - 04
Copy result
Copy all info as plain-text for tickets, reports, log lines.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
exportclipboardno oracledeterministicclient-side✓ · copiedready · done✓ Copied
shipped · ready to shipsuccess
Time math without DST landmines
Always store UTC
Convert to local for display only. UTC storage avoids DST jumps and timezone drift in databases.
Watch leap seconds
POSIX time doesn’t count leap seconds; some systems do. Don’t use timestamps for sub-second precision across boundaries.
Prefer Temporal eventually
The Date API has many quirks. Temporal is shipping in modern engines and handles zones correctly.
Cron is timezone-tricky
Cron expressions run in the daemon’s local zone unless told otherwise. Pin a TZ explicitly for predictable runs.
ISO 8601 sorts naturally
YYYY-MM-DDTHH:mm:ss sorts correctly as a string. Don’t use US-style MM/DD/YYYY for anything machine-readable.
Beware Y2K38
Signed 32-bit Unix timestamps overflow in 2038. Use 64-bit storage end-to-end for long-lived systems.
When to compute week numbers
Reporting, scheduling, planning.
Sprint planning
Reference sprints by ISO week (Sprint 2026-W21). Math runs in UTC under the hood to dodge DST jumps — local time is only for display. That means scheduling logic and date diffs survive March / November fall-back without the classic off-by-one-hour bug.
DST-safeReports
Weekly metrics aligned to ISO weeks. Browser clock is the only source — no external time server, no privacy leak, no rate limit. Pair with NTP-synced server time for high-accuracy use cases, but for human-scale work the browser clock is plenty.
UTC-correctEditorial calendars
Plan content drops by week number. ISO 8601 output sorts naturally as strings, perfect for filenames (report-2026-01-15.pdf) and log lines that need chronological ordering without parsing. Drop into S3 keys for free time-bucketing.
ISO-sortableFile naming
Reports-2026-W21.pdf for chronological sort. Pairs cleanly with cron, scheduled tasks, and timestamped log analysis. The tool's output drops directly into APScheduler, Linux cron, or GitHub Actions schedule expressions without re-formatting.
spec-gradePayroll
Many systems use ISO weeks for periods. Output respects the chosen timezone offset (Z vs +05:30) so REST APIs and database TIMESTAMP WITH TIME ZONE columns round-trip cleanly. No ambiguity between "noon UTC" and "noon local".
zone-awareInternational collab
ISO is the global default outside the US. Confirms the difference between epoch seconds (10 digits) and milliseconds (13 digits) before you commit code that mis-multiplies by 1000 — a classic bug in cross-language time handling.
epoch-accurateFrequently asked questions
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