Add or subtract days from a date, or measure the span between two dates.
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).
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.
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.
The day count is exact. Months and years are approximate because calendar months vary in length; for exact calendar age use the Age Calculator.