How to use
- Days between dates: pick a start date and an end date to see the total number of days, the same span in weeks and days, and in years, months and days.
- Add or subtract time: pick a start date, enter a whole number, choose Days, Weeks, Months or Years, and choose After or Before to get the resulting date.
- Countdown (D-day): pick a target date to see how many days are left (D-12), whether it is today (D-Day), or how many days have passed (D+3).
- All dates start as today’s date on your device, results update as soon as you change a value, and every resulting date is shown with its day of the week.
How days are counted
The total is the end date minus the start date, so the start date itself is not counted. From January 1 to January 10, 2025 is 9 days. If you turn on “Count the start date as day 1”, both dates are counted and the result is 10 days.
Weeks and days divide the total by 7: 10 days is 1 week and 3 days. Years, months and days count the largest number of whole months that fit, then the remaining days: January 31 to March 1, 2023 is 1 month and 1 day.
If the end date is before the start date, the two dates are swapped and the calculator tells you so.
Worked examples
- Days between January 1, 2026 and December 25, 2026: 358 days, which is 51 weeks and 1 day, or 11 months and 24 days.
- Age in days: from May 15, 1990 to October 20, 2026 is 13,307 days, or 1,901 weeks exactly, or 36 years, 5 months and 5 days.
- 100 days after Wednesday, January 1, 2025 is Friday, April 11, 2025. Counting January 1 as day 1, day 100 is Thursday, April 10, 2025.
- 1 month after January 31, 2024 is February 29, 2024, because 2024 is a leap year; 1 month after January 31, 2025 is February 28, 2025.
- Countdown: on October 20, 2026, the countdown to December 25, 2026 shows D-66.
What does “Count the start date as day 1” mean?
Some anniversaries and periods, such as day 100 of a relationship or a 30-day plan, count the first day itself as day 1. With this option, day 100 falls 99 days after the start date.
In Days between dates, the option adds 1 to the total, which is the same as counting both the start and end dates. In Add or subtract time, it applies only to Days and Weeks; months and years always move to the same day of the month.
Calendar rules
- Dates follow the Gregorian calendar, including February 29 in leap years. The calendar is applied to all dates, even those before it was adopted in 1582.
- Supported dates run from January 1, year 1 to December 31, 9999. Results outside this range show a message instead of a date.
- When the same day does not exist in the target month, such as one month after January 31, the last day of that month is used.
- “Today” in the countdown is the date set on your device, so it follows your own time zone.
Frequently asked questions
How many days are there between two dates?
Choose the two dates in Days between dates; the total is the end date minus the start date. For example, January 1 to January 10, 2025 is 9 days, or 10 days if you count the start date as day 1.
Does the day count include the start and end dates?
By default it counts the end date but not the start date. Turn on “Count the start date as day 1” to include both dates, which adds 1 to the total.
How many days until Christmas?
Pick December 25 as the target date in Countdown (D-day) to see the days left. For example, on October 20, 2026 it shows D-66.
How do I find the date 100 days from today?
In Add or subtract time, keep today as the start date, enter 100, and choose Days and After. For example, 100 days after January 1, 2025 is April 11, 2025.
What is one month after January 31?
The last day of February: February 29 in a leap year such as 2024, and February 28 otherwise. When the target month has no matching day, the calculator uses its last day.
How is the countdown calculated?
It is the number of days from today to the target date. Tomorrow is D-1, today is D-Day, and yesterday is D+1.
Are leap years included?
Yes. Calculations follow the real Gregorian calendar, including February 29 in leap years.