Counting days without a calculator
Sometimes you need to know how many days lie between two dates: for a deadline, a trip, a notice period or a countdown. A calculator is quick, but the manual method is easy to learn and helps you check any result. This guide shows two reliable methods and the pitfalls to avoid.
First, learn the month lengths
Both methods need the number of days in each month. January, March, May, July, August, October and December have 31 days. April, June, September and November have 30. February has 28, or 29 in a leap year. A knuckle trick helps: make a fist and count months along your knuckles and the gaps between them. Every month that lands on a knuckle has 31 days, and every month that lands in a gap has fewer.
Method 1: remaining days, full months, final days
- Count the days left in the start month: days in that month minus the start day.
- Add the full length of every month in between.
- Add the day number of the end date.
Example: from 15 March 2026 to 10 October 2026.
Days left in March: 31 minus 15 = 16.
Full months April to September: 30 + 31 + 30 + 31 + 31 + 30 = 183.
Days in October up to the 10th: 10.
Total: 16 + 183 + 10 = 209 days.
Method 2: day-of-the-year numbers
Convert each date to its day number in the year, then subtract. In a normal year, 1 March is day 60, 1 April is day 91, 1 July is day 182 and 1 October is day 274. In a leap year, every date from 1 March onward has a day number one higher.
Example: 15 March 2026 is day 74 (31 + 28 + 15). 10 October 2026 is day 283 (273 days to the end of September, plus 10). 283 minus 74 = 209 days, the same answer.
Crossing a year boundary
If the dates are in different years, count the days left in the start year and add the day number of the end date in the new year. A simpler way is Method 1: continue through December and into January.
Example: from 20 December 2026 to 10 January 2027: days left in December are 31 minus 20 = 11, plus 10 days in January = 21 days.
Leap years
If the period includes 29 February, add one day for it. For instance, from 20 February 2028 to 5 March 2028, the days left in February are 29 minus 20 = 9 (2028 is a leap year), plus 5 days in March = 14 days. Not sure whether a year is a leap year? Our guide to leap day birthdays explains the rule.
Should you include the end date?
The methods above count the days that have passed from the start date to the end date, so the start date is not counted. If both the first and the last day should be counted, such as a booking that covers both dates, add 1. Contracts and policies differ, so check which rule applies.
Turning days into weeks
Divide the number of days by 7. For 209 days, that is 29 weeks and 6 days, because 29 times 7 is 203, and 209 minus 203 is 6.
Check your result
The date difference calculator shows the total days, weeks, hours, a years/months/days breakdown, and how many of the days are weekdays. It also has an option to include the end date, so you can compare it with your hand calculation.
This guide is for general information. For legal or contractual deadlines, follow the counting rule that applies to you.