Julian Day Calculator: The Astronomer’s Clock
The Julian Day Number (JDN) counts days since noon on January 1, 4713 BC. It’s the standard for astronomers, historians, and calendar nerds.
How It Works
Formula:
JDN = (1461 × (Y + 4800 + (M - 14)/12))/4 + (367 × (M - 2 - 12 × ((M - 14)/12)))/12 - (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D - 32075
Key Milestones
| Event | Date | JDN |
|---|---|---|
| Unix Epoch | Jan 1, 1970 | 2440588 |
| Moon Landing | Jul 20, 1969 | 2440406 |
| Y2K | Jan 1, 2000 | 2451545 |
| Today (2025) | Jan 1, 2025 | 2460679 |
Fun Facts
- JDN starts at noon UT, not midnight
- Fractional part = time of day (0.5 = noon)
- Used in NASA, ESA, SETI
- Never changes — no leap seconds
Pro Tips
- Use JDN + 0.5 for midnight
- Pair with Day of Week
- Convert Julian Calendar dates separately
FAQs
Before 1582? Uses proleptic Gregorian.
Negative JDN? Yes — for ancient dates.
Accurate to second? Add fractional part.
Conclusion
Time is continuous. The Julian Day lets you measure it precisely — from Babylon to Mars missions.
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