Date Calculation Tips: Days, Weekdays and Age
Dates look simple until months have different lengths and leap years appear. These rules explain how date calculators get their answers.
Counting days between two dates
The usual answer is the end date minus the start date. From October 28 to November 27 is 30 days. If you want to count both the first and the last day, add 1. Contracts and rental periods sometimes use that inclusive count, so check which one applies.
Weekdays only
A 30-day span contains about 4 weeks and 2 days, and the number of Monday to Friday days depends on which weekdays the extra days fall on. Counting weekdays does not remove public holidays, because those differ by country.
Leap years
A year is a leap year when it is divisible by 4, except years divisible by 100 that are not divisible by 400. So 2024 and 2000 are leap years, but 1900 and 2100 are not. A leap year has 366 days.
Adding months
Months have different lengths, so adding a month to the 31st lands on a day that may not exist. Calculators usually clamp to the last day of the target month: January 31 plus one month is February 28 (or 29 in a leap year).
Working out exact age
Subtract the birth year from the current year, then subtract one if the birthday has not happened yet this year. Someone born on May 17, 1990 is 36 on October 28, 2026, and exactly 36 years, 5 months and 11 days old.
Countdowns
A countdown is the same subtraction with a future date. Check that you and the other person are using the same time zone when the date is close.