View a year as ISO weeks
ISO 8601 weeks always start Monday. Years can have 52 or 53 weeks depending on which day Jan 4 lands on. See the whole calendar at a glance.
How ISO weeks work
ISO 8601: every week starts Monday. Week 1 contains January 4. A year may have 52 or 53 ISO weeks. The last week of one year can spill into the next.
- 01
Find Jan 4
Whichever week contains it is Week 1.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
w1ISO 8601RFC 3339epochliveJan 4 · timestampanchor · ISOJan 4 in W1
loaded · ISO 8601zone · ready - 02
Step in 7s
Every Monday after starts a new ISO week.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via
Intl.DateTimeFormat.weeksUTC mathIntlDST-safelive+7d · optionsMon · liveMon, Mon, Mon…
options applied · UTCDST-safe - 03
Decide last week
If Dec 28 lands inside the calendar year, that week is still part of it.
Output respects the chosen timezone offset (
Zvs+05:30) and pads digits per ISO conventions so sorts stay chronological.edgeoffsetzone-awaresortablepaddededge · computedDec 28 · readyW52 or W53
computed · instantsortable - 04
See all weeks
The grid prints every week with its month and date range.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
gridclipboardno oracledeterministicclient-side✓ · copiedready · doneJan W1 1-7
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 use ISO weeks
Sprints, payroll, planning.
Sprint codes
W21 is sprint week 21 — same convention as Atlassian. 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-safePayroll
Some countries pay by ISO week. 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-correctReporting
Weekly KPIs aligned to ISO weeks. 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-sortableDate math
ISO week + day uniquely identifies a date. 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-gradeCapacity planning
Plan by 52/53 weeks rather than 12 months. 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-awareEU manufacturing
Week-of-year is the dominant planning calendar. 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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