What day number is today? The day number calculator shows how many days have passed since January 1, Year 1 (the start of the Common Era).
How the day number works
The calculator uses the proleptic Gregorian calendar, which extends our modern calendar backwards in time. January 1, Year 1 CE is day 1. The formula accounts for leap years using the standard Gregorian rules: divisible by 4, except centuries (unless divisible by 400).
Features
- Date to day number - Enter any date, see its day number since Year 1
- Day number to date - Enter a day number, get the calendar date
- Multiple epoch options - Switch between Year 1, Julian Day, Unix, Excel, and J2000
- Stats breakdown - See the equivalent in years, weeks, and hours
Why day numbers matter
Day numbering systems simplify date math. Instead of juggling months with different lengths and leap year rules, you can subtract two day numbers to get the exact days between them. Astronomers use Julian Day Numbers, programmers use Unix timestamps, and spreadsheet users encounter Excel serial dates. (Yes, even you Google Sheets aficionados).
Related calculators
- Julian Day Calculator - The astronomer's day numbering system
- Unix Day Calculator - Days since January 1, 1970
- Excel Date Serial Calculator - Spreadsheet date numbers
- J2000 Day Calculator - Modern astronomical epoch
- Days Since Date Calculator - Days from any custom date
Easier than counting, isn't it?
