Julian Date Converter
Convert between Julian day numbers and Gregorian calendar dates.
Enter in any format (e.g. 1/1/1900, today)
Enter the Julian Date to convert to UTC
Results
Enter values and click Calculate to see results
Methodology
This calculator uses the standard astronomical algorithms for converting between Gregorian calendar dates and Julian dates. The Gregorian to Julian conversion uses the algorithm: JD = JDN + time_fraction - 0.5, where JDN is calculated using the standard formula accounting for leap years and calendar reforms. The Julian to Gregorian conversion uses the inverse algorithm to extract year, month, day, and time components.
Important Disclaimers:
- This calculator uses UTC (Coordinated Universal Time) as the time standard
- Julian dates are calculated with 6 decimal places (≈ 0.1 second precision)
- For high-precision astronomical work, consider leap seconds and time scale differences
- The calculator is accurate for dates from 4713 BCE to far future dates
- Results assume the proleptic Gregorian calendar for dates before 1582
- For historical dates, be aware of calendar system differences in different regions
About This Calculator
1. What is a Julian Date and why is it used?
Julian Date (JD) is a continuous count of days since January 1, 4713 BCE in the proleptic Julian calendar. It was introduced by Joseph Justus Scaliger in 1583 and is widely used in astronomy because it eliminates the complications of varying month lengths, leap years, and different calendar systems. It provides a simple way to calculate time intervals and is essential for astronomical calculations.
2. Why does Julian Date start at noon instead of midnight?
Julian Date starts at noon Universal Time because it was designed for astronomical use. Historically, astronomers worked through the night and wanted to avoid having the date change in the middle of their observations. By starting the day at noon, a single Julian Date covers an entire night of observation from evening to dawn.
3. What is the difference between Julian Date variants (JD, MJD, RJD, TJD)?
Julian Date (JD) is the standard continuous day count. Modified Julian Date (MJD = JD - 2400000.5) starts at midnight and uses smaller numbers. Reduced Julian Date (RJD = JD - 2400000) is similar to MJD but starts at noon. Truncated Julian Date (TJD) is the integer part of (JD - 2440000.5) and is used in some satellite systems. Each variant serves different purposes but represents the same moment in time.
4. How accurate is this Julian Date calculator?
This calculator provides Julian Dates with 6 decimal places, giving precision to about 0.1 seconds. The algorithms used are accurate for dates from 4713 BCE to far future dates. For most astronomical and scientific applications, this precision is more than sufficient. For applications requiring higher precision, consider leap seconds and the difference between UTC and Terrestrial Time (TT).
5. What date formats can I enter for UTC dates?
The calculator accepts various date formats including ISO format (2024-01-01), US format (1/1/2024), European format (1.1.2024), and natural language like 'today' or 'now'. You can also include time components like '2024-01-01 12:30:00'. The calculator will attempt to parse most reasonable date formats automatically.
6. What are some important Julian Date epochs?
Key epochs include: J2000.0 (JD 2451545.0) - the standard astronomical reference epoch; Unix Epoch (JD 2440587.5) - start of Unix time; Y2K (JD 2451544.5) - January 1, 2000 at midnight; GPS Epoch (JD 2444244.5) - start of GPS time. These epochs are fundamental reference points for various time systems and astronomical calculations.