Date calculation guides

The calculator gives you the number. These guides explain the rules behind it, with examples you can check in the calculator yourself.

• How to count the days between two dates (and when to add 1)Subtract the earlier date from the later one, and add 1 only if both the first and last day count. Examples for countdowns, trips, leap years and spreadsheets.
• How to count business days and find a deadlineA business day is a weekday that isn't a holiday. How to find the date 5 or 10 business days away, whether today counts, and which US holidays are skipped.
• US federal holidays on a weekend: which day is observedA federal holiday on a Saturday is observed the Friday before; on a Sunday, the Monday after. Observed dates for 2026 to 2028, including New Year's Day 2028 on Dec 31, 2027.
• Adding months to a date: what happens to January 31January 31 plus one month is February 28 (29 in a leap year). How the end-of-month rule works, why adding a month twice differs from adding two, and how JavaScript differs.
• How to calculate your exact age in years, months and daysCount whole years to the last birthday, then whole months, then the days left. A worked example, birthdays at the end of a month, February 29 birthdays and age in days.
• ISO week numbers: why January 1 can be in week 53ISO weeks start on Monday and week 1 holds the year's first Thursday, so January 1, 2027 is in week 53 of 2026. The rule, 53-week years, and why Excel's WEEKNUM differs.