Linked astronomy tools
Quick start (3 steps)
- Enter date / time / timezone.
- Click Compute.
- Copy share URL for logs and verification.
Inputs
Results
| JD (UTC) | — |
|---|---|
| MJD (UTC) | — |
| Unix time (s) | — |
| ISO UTC | — |
| ISO local | — |
| Julian Day Number (JDN) | — |
| Resolved TZ | — |
This tool runs in your browser. Inputs are not sent.
Calculation steps
JD = UnixSeconds / 86400 + 2440587.5, MJD = JD - 2400000.5
Interpretation & notes
- JD is continuous time; JDN is integer day number.
- J2000.0 (Terrestrial Time (TT)) and UTC-based JD are not the same value.
- When UTC offset is set, it has priority over IANA timezone.
Abbreviation legend
- JD: Julian Date.
- MJD: Modified Julian Date.
- UTC: Coordinated Universal Time.
- IANA: standard timezone ID (for example, Asia/Tokyo).
- Julian date (Julian Date and Modified Julian Date (JD/MJD)) calculator: Julian date conversion (this page).
- Sidereal time (GMST/LST) calculator: Sidereal time (Greenwich Mean Sidereal Time and Local Sidereal Time (GMST/LST)).
- RA/Dec to Alt/Az calculator: Right Ascension and Declination (RA/Dec) to Altitude and Azimuth (Alt/Az) conversion.
- Angular separation and position angle (PA) calculator: Angular separation + position angle.
- Twilight, sunrise, and sunset calculator: Twilight, sunrise and sunset checks.
- Observability time plotter: Observability window charts.
References
How to use this calculator effectively
Use this page to convert a civil date and time to Julian Date (JD) and Modified Julian Date (MJD), or to compare astronomy timestamps without local calendar ambiguity.
How it works
The calculator normalizes the entered date, time, and time zone to a continuous day count. JD starts at noon UTC, while MJD subtracts 2,400,000.5 so the value is shorter for modern observations.
When to use
Use it for observing logs, ephemeris checks, time-series labels, and matching timestamps between astronomy tools that use JD or MJD instead of local calendar dates.
Common mistakes to avoid
- Forgetting that JD changes at noon UTC rather than local midnight.
- Mixing local time and UTC without setting the correct offset.
- Comparing JD and MJD values without the 2,400,000.5 offset.
- Dropping fractional days when time-of-day precision matters.
Interpretation and worked example
Run one date-time as a baseline, then adjust only the time zone or time of day if a source uses UTC. A difference of 0.5 day means 12 hours, not a calendar error.
See also
FAQ
What is the difference between Coordinated Universal Time (UTC), Terrestrial Time (TT), and International Atomic Time (TAI)?
Coordinated Universal Time (UTC) is the civil time scale with leap seconds. International Atomic Time (TAI) is a continuous atomic scale, and Terrestrial Time (TT) is defined as TAI + 32.184 seconds for ephemeris calculations.
How are Julian Date (JD), Julian Day Number (JDN), and Modified Julian Date (MJD) different?
Julian Date (JD) is continuous time, Julian Day Number (JDN) is an integer day index, and Modified Julian Date (MJD) is JD minus 2400000.5.
Why is J2000.0 written as 2451545.0?
J2000.0 is defined at 2000-01-01 12:00 TT with JD 2451545.0. This page returns UTC-based values, so the apparent value can differ when the time scale assumption changes.
Why can local time display differ for the same instant?
Local representation changes with time zone rules. For the same instant, UTC and JD remain identical while only the local clock display changes.
Why do daylight-saving transitions cause input errors?
DST transitions create nonexistent times (gap) and duplicated times (fold). Use the UTC offset override when you need to pin the exact instant.
Which astronomy tool should I open next?
Use this page for JD/MJD conversion. Open sidereal time for GMST/LST, RA/Dec to Alt/Az for horizon coordinates, and angular separation for separation or position angle checks.