How to read A/B codes
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 (A1): Julian date conversion (this page).
- Sidereal time (GMST/LST) calculator (A2): Sidereal time (Greenwich Mean Sidereal Time and Local Sidereal Time (GMST/LST)).
- RA/Dec to Alt/Az calculator (A3): Right Ascension and Declination (RA/Dec) to Altitude and Azimuth (Alt/Az) conversion.
- Angular separation and position angle (PA) calculator (A4): Angular separation + position angle.
- Twilight, sunrise, and sunset calculator (A5): Twilight, sunrise and sunset checks.
- Observability time plotter (A7): Observability window charts.
References
How to use this calculator effectively
This guide helps you use Julian date (Julian Date and Modified Julian Date (JD/MJD)) calculator in a repeatable way: define a baseline, change one variable at a time, and interpret outputs with explicit assumptions before you share or act on results.
How it works
The page applies deterministic logic to your inputs and shows rounded output for readability. Treat it as a comparison workflow: run one baseline case, adjust a single parameter, and measure both absolute and percentage deltas. If a result seems off, verify units, time basis, and sign conventions before drawing conclusions. This approach keeps your analysis reproducible across teammates and sessions.
When to use
Use this page when you need a fast estimate, a classroom check, or a practical what-if comparison. It works best for planning and prioritization steps where you need direction and magnitude quickly before investing in deeper modeling, manual spreadsheets, or formal external review.
Common mistakes to avoid
- Changing multiple parameters at once, which hides the true cause of output movement.
- Mixing units (percent vs decimal, monthly vs yearly, gross vs net) across scenarios.
- Comparing with another tool without aligning defaults, constants, and rounding rules.
- Using rounded display values as exact downstream inputs without re-checking precision.
Interpretation and worked example
Run a baseline scenario and keep that result visible. Next, modify one assumption to reflect your realistic alternative and compare direction plus size of change. If the direction matches your domain expectation and the size is plausible, your setup is usually coherent. If not, check hidden defaults, boundary conditions, and interpretation notes before deciding which scenario to adopt.
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.
What do A1 through A4 mean?
A1 is Julian Date and Modified Julian Date (JD/MJD) conversion, A2 is sidereal time (GMST/LST), A3 converts right ascension and declination (RA/Dec) to altitude and azimuth (Alt/Az), and A4 calculates angular separation and position angle (PA).