Two ways to calculate
Between two dates counts how many days, months and years lie between a start date and an end date. Use it when you need the length of a period, such as length of service, a contract term or time served.
Add or subtract finds the date a number of days, weeks, months or years before or after a reference date. Use it when you need a specific date, such as a contract end date, a submission deadline or the date 100 days from now.
Months are counted on the calendar
Months are counted by calendar date, not in 30-day blocks. March 15 to September 15 is exactly 6 months, and one day short of that, September 14, is 5 months and 30 days. If the resulting month has no matching day, the last day of that month is used. The period calculation and date addition use the same rule, so their results always agree.
Legal deadlines may be counted differently
Civil law or specific laws often set their own rules for counting periods, such as whether the first day is included. For dates with legal consequences, such as severance pay, leave or court deadlines, double-check with the relevant authority or a professional.