How to Use This Calculator
To count down to an event:
Select today as the start date, and the date you're waiting for as the end date. The time difference shown is how much time is left.
To check the duration between two dates:
Enter the date that begins a period as the start date (e.g., the first day of a course), and the date that ends it as the end date.
To add or subtract days:
Use the second calculator — pick a start date, enter an amount and unit (days, weeks, months, or years), and choose whether to add or subtract. The resulting date will be calculated instantly.
How to Calculate Days Between Two Dates Manually
To calculate the days between two dates yourself — say, 4 September 1999 and 2 January 2003 — you can follow these steps:
1. Count the full years between the dates.
Add as many full years as possible to the earlier date without passing the later date, then subtract to find the year difference. Remember to account for any leap years in that span.
2. Count the full months left over.
Work out how many complete months remain after the full years are counted, using the actual number of days in each month.
3. Count the remaining days.
Add the leftover days in the start month to the day-of-month of the end date.
4. Add the results together to get the total number of days between the two dates. If you don't want to include the end date itself, subtract 1 from the final total.
It's a fair bit of manual arithmetic — which is exactly why a calculator like this one is faster and avoids simple counting mistakes, especially around leap years.
Common Applications
People use a date calculator for all kinds of everyday situations, including:
- Checking how many days are left until a deadline or exam
- Working out the duration of an event, like how long a school term runs
- Calculating age in years, months, or days
- Counting down to a wedding, vacation, or birthday
- Finding out how many days have passed since a past event
- Working out which day of the week a future or past date falls on
- Adding or subtracting time from a date — e.g., "what date is 90 days from today?"
The Short History of Conventional Time
Long before calculators existed, most cultures tracked time using lunar phases and the changing seasons. The word "calendar" comes from the Latin calendae, marking the first day of the Roman month at the new moon.
When Julius Caesar reformed the Roman calendar in 46 BC, he replaced a drifting 355-day lunar calendar with a 365.25-day solar calendar inspired by Egyptian astronomy, adding a leap day every four years to keep things aligned.
The Julian calendar was still slightly imprecise — off by about 11 minutes a year — which slowly caused dates to drift. Pope Gregory XIII corrected this in 1582 with the Gregorian calendar, which skips leap years in century years not divisible by 400. This is the calendar most of the world uses today.
Frequently Asked Questions
How do I know if a year is a leap year?
Divide the year by 4 — if it doesn't divide evenly, it's not a leap year. If it does, divide by 100; if that also divides evenly, divide by 400. A year divisible by 400 is a leap year, but a year divisible by 100 (and not 400) is not.
Does the calculator include the end date in the count?
By default, the calculator counts full calendar days between the two dates. If you don't want the end date included, subtract 1 from the result.
Can I calculate dates far in the past or future?
Yes — both tools work with any valid calendar date, past or future, and automatically account for leap years across the full range.
What's the difference between the two calculators on this page?
The first finds the time difference between two dates you already know. The second takes one date and adds or subtracts a time period to find a new date.