Date Calculations: Days Between Dates and Age
Calendars are full of exceptions: months of different lengths, leap years, weekends, and clocks that change twice a year. Here is how to think about the most common date calculations without getting caught out.
Counting days between two dates
The simplest approach is to convert both dates to a day number and subtract. The only question is whether to count the end date. Between 1 and 2 January the difference is one day, but a hotel stay from 1 to 2 January is one night and two calendar days. Our date difference calculator has an option for inclusive counting.
Years, months and days
Because months have different lengths, βone monthβ is not a fixed number of days. Most tools, including ours, add whole months to the start date first and then count the remaining days. From 31 January to 1 March in a leap year, that gives one month and one day.
Leap years
A year is a leap year if it is divisible by 4, except for century years not divisible by 400. So 2024 and 2000 were leap years, while 1900 and 2100 are not. People born on 29 February usually celebrate on 28 February or 1 March in other years; the age calculator uses 28 February.
Business days
A working week is usually Monday to Friday, so 14 calendar days contain 10 business days. Public holidays vary by country, so subtract them yourself for precise planning, such as legal notice periods or delivery estimates.
Pregnancy dating
Doctors count pregnancy from the first day of the last period, and a full term is 40 weeks (280 days). The due date calculator applies that rule and adjusts for cycle length.
Time zones and daylight saving
When scheduling across regions, always specify the time zone and the date. The United States and Europe change their clocks on different weekends, so the gap between New York and London varies for a few weeks each year. Some places use unusual offsets such as UTC+5:30 for India. The time zone converter looks up the right offset for the exact date.
Counting down to an event
Use the countdown timer to follow days, hours and seconds to a holiday or deadline, and share the link with others.
Good habits
- Write dates unambiguously, for example 2026-10-03 or 3 October 2026, instead of 03/10/26.
- Store and compare times in UTC when building software.
- Be explicit about whether ranges include their end date.