Add and subtract durations
Type expressions like 2h30m + 1h15m - 45s. We parse durations, apply the signs and emit the breakdown plus total seconds.
3h - 30m - 15s
2.5h + 250ms
How parsing works
The expression splits on bare + / - then each chunk is matched with a regex picking out number + unit. Signs alternate per separator.
- 01
Type chunks
Each chunk is one or more number+unit pairs.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
chunksISO 8601RFC 3339epochlivein · timestampchunk · ISO2h 30m
loaded · ISO 8601zone · ready - 02
Operator splits
+ / - between chunks alternate sign for the next chunk.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via
Intl.DateTimeFormat.opUTC mathIntlDST-safelive+ · optionsnext · live+ 1h 15m
options applied · UTCDST-safe - 03
Normalize
Everything converts to seconds before summation.
Output respects the chosen timezone offset (
Zvs+05:30) and pads digits per ISO conventions so sorts stay chronological.normoffsetzone-awaresortablepaddednorm · computedsec · ready9000 + 4500
computed · instantsortable - 04
Reformat
Total seconds → days/hours/minutes/seconds for humans.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
formatclipboardno oracledeterministicclient-side✓ · copiedready · done3h 44m 15s
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 add durations
Logs, ETA, scheduling.
ETA math
How long until everything finishes? 30m + 45m + 15m. 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-safeCooking
Stack timers for a multi-step recipe. 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-correctProject hours
Sum task durations from a planner. 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-sortableTest runs
Total of multiple suite durations. 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-gradeMusic sets
Plan a DJ set by adding track lengths. 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-awareTravel legs
Multi-leg journey time = sum of legs. 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
Something missing?
Tell us what you need. We ship the top requests every two weeks.