How dates are counted
This tool uses the real calendar, not a 30-day month shortcut. February has 28 days, or 29 in a leap year. A leap year is a year divisible by 4, except a century year must be divisible by 400. So 2000 was a leap year and 1900 was not. The difference is built from JavaScript Date objects so month lengths stay correct.
How to use it
Enter a start date. Leave the second box empty to use today, or pick an end date for a contract, a sea-time period or an age at a given day. Years, months and days are the broken-down age. Total days is the exact number of midnights between the two dates.
Who should use it
Useful for age checks, document validity and simple duration counts. Official sea-time still follows the CDC / SID and the company’s service letters.