How the calculation works
The calculator uses the calendar borrowing method to express the exact duration in Y:M:D format. When the end-date day is less than the start-date day, one month is borrowed (using the actual length of the previous month). This matches the same method used by clinical and legal date arithmetic.
Total units (total days, hours, etc.) are calculated as cumulative sums from the millisecond difference, not converted from Y:M:D — so they are independent, precise measurements.
Common use cases
- Anniversaries: How long have you been married, together, or at your job?
- Project timelines: How long did a project take, or how long until a deadline?
- Legal & financial: Calculating exact holding periods, notice periods, or contract durations.
- Historical: How long ago did a historical event occur?
- Pregnancy: How many days between LMP and due date?