Why can a calendar month have different numbers of days?

Calendar months range from 28 to 31 days, so the years-months-days result uses actual month lengths rather than treating every month as a fixed duration.

Duration Between Dates

Compare two dates as total days, calendar units and complete weeks.

01 / INPUTS

Start dateEnd date

02 / RESULT

Total days: 60 days
Calendar duration: 0 years, 2 months, 0 days
Complete weeks: 8 weeks

Calculation trace

2024-01-01 to 2024-03-01 = 60 days
60 days
Total days
0 years, 2 months, 0 days
0y 2m 0d
Calendar duration
floor(60 ÷ 7) = 8 weeks
8 weeks
Complete weeks

How it works

The tool parses YYYY-MM-DD ISO calendar dates and computes total days from their UTC-midnight difference divided by 86,400,000 milliseconds. It also reports calendar years, months and days, and complete weeks as floor(total days ÷ 7).

Assumptions & limits

Uses calendar dates only, without times of day or user time zones.
The end date must be on or after the start date, and complete weeks exclude any leftover days.

FAQ

Why can a calendar month have different numbers of days?: Calendar months range from 28 to 31 days, so the years-months-days result uses actual month lengths rather than treating every month as a fixed duration.

Related calculators

SOURCES

International Organization for Standardization (ISO)
ISO 8601 — Date and time format ↗
Calendar date representation used to parse dates and compute civil year/month/day and total-day differences on the proleptic Gregorian calendar
Accessed 2026-08-19