Unix time and Julian Date both answer the same quiet question: “what number represents this moment?” Software keeps timestamps as seconds since 1970; astronomy and satellite work keep dates as continuous day counts. The Unix Time Converter turns epoch seconds into UTC dates and back, while the Julian Date Converter produces JD, MJD, RJD, and TJD day counts from a UTC civil date — and reverses the conversion. One is the lingua franca of logs and APIs; the other is the day-scale count astronomers and data feeds rely on.
What each calculator does
The Unix Time Converter has two modes. Timestamp to UTC date multiplies the entered Unix seconds by 1000 to get milliseconds and constructs the UTC instant from the epoch, 1970-01-01 00:00:00 UTC — so 1700000000 displays as 2023-11-14 22:13:20 UTC, with the ISO 8601 string and the milliseconds value shown alongside. UTC date to timestamp parses the entered date text (treating a missing offset as UTC) and truncates milliseconds divided by 1000 to whole seconds, so fractional seconds are not rounded up. The page’s reference table anchors the scale: timestamp 0 is the epoch, 1 is one second later, and 86400 is one ordinary day later.
The Julian Date Converter accepts a calendar date and an HH:mm time interpreted as UTC and returns four related counts: Julian Date (JD), Modified Julian Date (MJD = JD − 2400000.5), Reduced Julian Date (RJD = JD − 2400000), and Truncated Julian Date (TJD = JD − 2440000.5), each displayed to six decimal places. JD advances by exactly one per day, with the fraction holding the time of day, and midnight UTC lands on a .5 because JD starts at noon UTC by convention. The reverse mode converts a decimal JD back to a UTC civil timestamp. The calculator applies the Julian calendar through October 4, 1582 and the Gregorian calendar from October 15, 1582, and accepts civil years 0001 through 9999. In the worked example, 2023-02-25 00:00 UTC becomes JD 2460000.500000, MJD 60000.000000, RJD 60000.500000, and TJD 20000.000000.
Side-by-side
| Unix Time Converter | Julian Date Converter | |
|---|---|---|
| What the number counts | Seconds since 1970-01-01 00:00:00 UTC | Days from the astronomical JD epoch, with a fraction for time |
| Typical magnitude in the 2020s | About 1.7 billion seconds | About 2.46 million days |
| Primary result | UTC date and time (plus ISO 8601 and milliseconds) | JD, MJD, RJD, and TJD at six decimals |
| Time precision | Whole seconds (truncated on input) | Minutes entered; seconds set to zero |
| Calendar handling | UTC-anchored date parsing | Mixed Julian/Gregorian branch, 1582 reform, years 0001–9999 |
| Day rollover | At midnight UTC (epoch seconds) | At astronomical noon for JD; MJD rolls at midnight |
| Typical users | Developers, analysts, log and API pipelines | Astronomers, satellite operators, data feeds that request JD forms |
When to use which
Use the Unix Time Converter when your data lives in software: log lines, API payloads, database rows, cache expirations, message queues, or cron output. Numbers in seconds since the epoch sort, compare, and index cleanly, and the converter’s UTC-first behavior keeps a global instant stable across computers and time zones.
Use the Julian Date Converter when the receiving system explicitly wants an astronomical day count — observation logs, ephemerides, geodesy, or a data feed that names JD, MJD, RJD, or TJD. A useful practice both pages support is keeping both forms: the numeric value for calculation and an ISO-style UTC timestamp for human review. If no system demands a particular format, Unix time is the simpler default for general-purpose records.
Limits and disclaimer
Both calculators are educational tools for planning and log work, not time-service references. The Unix Time Converter truncates fractional seconds in date-to-timestamp mode, treats offset-less date text as UTC (some programming libraries assume local time instead), and does not display leap seconds such as 23:59:60. The Julian Date Converter accepts minutes but not seconds in civil-date input, rejects year zero and BCE dates, and its pre-1582 calendar branch may not match the civil calendar used in a particular historical record; UTC also is not the same as terrestrial time or barycentric dynamical time for professional ephemeris work.