Date Duration Calculator
Count the days, weeks, and months between two dates.
Inputs
Total Days
364
Weeks
52.00
Months (approx)
11.96
Years (approx)
0.997
Step by step
Compute millisecond difference and convert to whole days
(December 31, 2025 − January 1, 2025) ÷ 86 400 000
= 364 days
Divide by 7 for weeks
364 ÷ 7
= 52.00 weeks
Divide by 30.437 for approximate months
364 ÷ 30.437
= 11.96 months
Uses the average Gregorian month length (365.25 ÷ 12). Approximate.
Divide by 365.25 for approximate years
364 ÷ 365.25
= 0.997 years
How it works
The number of days is the exact difference between the two dates. Weeks, months, and years are derived from that day count (months and years use average lengths, so they're approximate).
Formula
Days = (end timestamp − start timestamp) ÷ 86 400 000, rounded to the nearest whole day. Weeks = days ÷ 7. Months ≈ days ÷ 30.437. Years ≈ days ÷ 365.25.
- 30.437
- Average days per Gregorian month (365.25 ÷ 12)
- 365.25
- Average days per Gregorian year (accounts for leap years)
- 86 400 000
- Milliseconds in one day
Frequently Asked Questions
Does it count both the start and end day?
By default it counts the gap between the dates. Turn on 'Include End Day' to count the final day as well, which is useful for billing or event durations.
Are months exact?
The day count is exact. Months and years are approximate because calendar months vary in length; for exact calendar age use the Age Calculator.