Date Calculator

How this date calculator works

Use the first mode to find the number of days between two dates, or switch to the second mode to add or subtract days, weeks, months, or years from a starting date to find a resulting date.

How the calculations work

Days between two dates is a straightforward subtraction once both dates are converted to a common day count. Adding or subtracting months or years is slightly trickier than days or weeks, since months have different lengths, so the calculation shifts the calendar month and year directly rather than adding a fixed number of days.

Step-by-step: days between two dates

Convert both dates to a total day count from a fixed reference point.
Subtract the earlier date’s day count from the later date’s day count.
The result is the total number of days between them.

Worked example

From March 15th to July 23rd of the same year:

Days remaining in March: 16
Days in April, May, June: 30 + 31 + 30 = 91
Days into July: 23
Total: 16 + 91 + 23 = 130 days, or about 18 weeks, or roughly 4.3 months

Common mistakes

Forgetting to include or exclude the start date depending on what you’re calculating, some contexts count inclusively, others don’t.
Miscounting days in February during leap years, which adds an extra day every four years.
Adding 30 days per month as an approximation when calculating exact dates, months actually range from 28 to 31 days.

Common uses

Calculating a project deadline or contract end date.
Figuring out how many days until an event, trip, or milestone.
Working out an exact date a fixed number of days, weeks, or months in the past or future.

Does this calculator count the start date?

The days-between mode counts the actual number of calendar days separating the two dates. Some contexts (like a lease term) count inclusively, adding one extra day, so check which convention applies to your specific situation.

How are months calculated when adding or subtracting?

Adding a month shifts the calendar month directly rather than adding a fixed 30 days, so adding one month to January 31st typically lands on the last valid day of February, not exactly 30 days later.

Does this account for leap years?

Yes, calculations are based on actual calendar dates, so leap years are automatically factored in.

What’s the difference between this and the age calculator?

The age calculator is specifically built around calculating a person’s age from a birth date. This date calculator is more general-purpose, useful for any two dates or date math, not just age.