Date Calculator: Days Between Dates and D-Day
Count days until or since a date, find the days, weeks, months, and business days between two dates, and add or subtract days.
D-day countdown
Add or subtract days
Days between two dates
Anniversaries (100 days, 1,000 days)
How to count days between dates
- 1
Count down to a date
Enter a target date in D-day to see how many days are left or have passed.
- 2
Measure between two dates
Enter a start and end date to get days, weeks, months and days, and business days. Tick “Include end date” if both ends should count.
- 3
Add or subtract days
Pick a start date, choose add or subtract, and enter the number of days to get the resulting date and weekday.
Counting days correctly
Inclusive or exclusive
Most off-by-one errors come from not deciding whether to count both ends. From October 1 to October 10 is 9 days apart, but 10 calendar days if you count both the first and the last day. Hotel nights use the first number, a trip itinerary the second.
D-day notation
D-n counts the days left before the target date. On the target date it reads D-day, and afterwards D+n. If an exam is on November 13, then October 1 is D-43.
100th-day anniversaries
Korean 100-day celebrations for babies and couples count the start date as day 1. The 100th day is therefore the start date plus 99 days, and the 1,000th day is the start plus 999. The anniversary list uses this rule by default.
Months and leap years
Months have 28 to 31 days, so “2 months” is not a fixed number of days. When the start date is the 31st and the end month is shorter, the count rolls over to the 1st of the next month. 2024 is a leap year: January 1, 2024 to January 1, 2025 is 366 days.
Time zones
Dates are calendar days on your device. No clock time is involved, so daylight saving changes and UTC offsets never shift the result by a day.