How does TAI differ from UTC?

TAI is a continuous atomic time scale; UTC is TAI plus a varying integer number of leap seconds (currently 37 as of mid-2024) to keep it within 0.9 s of solar time. TAI never inserts leap seconds.

GPS time is a continuous scale that started at 00:00:00 UTC on 6 January 1980. It's exactly 19 seconds behind TAI and currently 18 seconds ahead of UTC because it doesn't include later leap seconds.

What's the difference between JD and MJD?

Julian Day counts days since noon UTC on 24 November 4714 BCE (proleptic Gregorian). MJD is JD − 2400000.5, so it starts at midnight on 17 November 1858 and produces shorter numbers for modern dates.

Does the tool need internet access?

No. All conversions, including the leap-second table baked into the code, run locally in your browser.

Unix · TAI · GPS · JD · MJD · J2000 · live

See cosmic time in seven forms

Unix, TAI, GPS, Julian Day, Modified Julian Day and J2000 — all ticking in real time. Useful for astronomy, satellites and historical timekeeping.

Quick Start

How cosmic time scales relate

Unix counts seconds from 1970. TAI adds the leap-second offset (37s today). GPS shifts from TAI by 19s and starts in 1980. JD/MJD/J2000 are astronomical day counts.

01
Browser ticks
Date.now() drives everything — 20 ticks/sec.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
tickISO 8601RFC 3339epochlive
now · timestampms · ISO
```
Date.now()
```
loaded · ISO 8601zone · ready
02
Apply leap seconds
TAI = Unix + 37 (as of 2024).
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via Intl.DateTimeFormat.
taiUTC mathIntlDST-safelive
Δ · options+37 · live
```
Unix + 37
```
options applied · UTCDST-safe
03
Shift to GPS
GPS = TAI - 19, starting from 1980-01-06.
Output respects the chosen timezone offset (Z vs +05:30) and pads digits per ISO conventions so sorts stay chronological.
gpsoffsetzone-awaresortablepadded
epoch · computed1980 · ready
```
TAI - 19 - 1980 offset
```
computed · instantsortable
04
Julian Day family
JD / MJD / J2000 all derive from Unix ms via fixed offsets.
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
astroclipboardno oracledeterministicclient-side
✓ · copiedready · done
```
JD 2460852.3
```
computed · ready to shipsuccess

Pro tips

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.

Use Cases

When to compare time scales

Astronomy, GNSS, history.

Astronomy

JD and J2000 are standard ephemeris coordinates. 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-safe

GPS payloads

Telemetry timestamps are GPS seconds — convert before logging. 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-correct

Leap-second debugging

Diff TAI vs Unix to confirm leap-second handling. 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-sortable

Scientific archives

MJD is preferred in radio astronomy data. 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-grade

Historical dates

JD covers BCE — handy for ancient dates. 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-aware

Teach time

See seven scales side-by-side as a learning aid. 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-accurate

FAQ

Frequently asked questions

Why is TAI offset 37s?

37 leap seconds have been inserted since 1972 — last in December 2016.

Will leap seconds be removed?

The CGPM voted to retire leap seconds by 2035. Until then we add them as announced.

Why ticks every 50ms?

Smooth enough to read while keeping CPU usage low.

JD vs MJD?

MJD = JD - 2400000.5 — modern shorter form that starts at midnight, not noon.

Is anything sent to a server?

No. All clocks come from your browser.

Suggest a tool

Something missing?

Tell us what you need. We ship the top requests every two weeks.

47 in queue·8 shipped this month

Request queued. Thanks!