Why Use This Tool?
Astronomers, historians, and software loggers share one habit: they count days continuously instead of juggling months and leap years. The Julian day number does exactly that — one unbroken count since January 1, 4713 BC. This converter swaps calendar dates and Julian days in both directions.
Enter a time of day in Universal Time and the converter produces the full decimal Julian date, not just the whole number. Paste a Julian day back in and you get the exact Gregorian date. The math is the standard Gregorian algorithm, verified against known reference values.
How It Works
- Pick a calendar date (and optionally a time in UT) to get its Julian day.
- Or paste a Julian day number to decode it back to a date.
- Use the JDN for date-only work; use the decimal Julian date for sub-day precision.
Frequently Asked Questions
What is a Julian day number?
A continuous count of days since January 1, 4713 BC in the proleptic Julian calendar — a favorite in astronomy because adding or subtracting days needs no calendar math.
How do I convert a date to a Julian day?
Enter the date above and read the JDN. With a time of day entered in UT, you also get the decimal Julian date, where each 0.5 step is twelve hours.
Why does the Julian day start at noon?
By convention the Julian day increments at 12:00 UT, so astronomers observing at night work within a single day number. That's why the tool asks for the time separately.
Tips & Things to Watch
- JDN 2451545 is exactly January 1, 2000, 12:00 UT — a handy sanity check.
- For ordinal dates (YYYY-DDD style), use the Day of Year calculator instead.
- Julian days ignore time zones entirely — always note UT in your logs.