Solo Biz Soft Editorial Team Counting days between dates comes up for deadlines, holidays, contracts and invoices. You can do it by hand with a simple method, or let a calculator do it. Here is how it works.
The formula
Days between = later date − earlier date. Count the days left in the first month, add the full months in between, then add the days in the last month.
Step by step
- Count the days remaining in the first month.
- Add the total days in every full month in between.
- Add the day number in the final month.
- Decide whether to include the end date, and add 1 if you do.
Worked example
From 20 September to 31 October: 10 days left in September (30 − 20) plus 31 days in October = 41 days.
Common mistakes
- Forgetting that months have different lengths
- Missing 29 February in a leap year
- Counting both the first and last day by accident
Skip the maths: use our free days between dates calculator. To add days to a date, use the date calculator.
Frequently asked questions
How many weekdays are in a period?
Count the weeks and multiply by five, then handle the remaining days by hand, or use a calculator.
Do I include the start date?
It depends on the context. Contracts often say. If not, decide and be consistent.
How do leap years work?
A year divisible by 4 is a leap year, except most century years unless divisible by 400.
More free tools: browse all our free calculators.
This article is general information only.