What Is a Date Calculator?
A date calculator (or days between dates calculator) finds the number of days, weeks, months, and years between two dates, or it can add or subtract a certain amount of time from a starting date. It is useful for counting down to events, calculating age, planning projects, tracking deadlines, figuring out billing periods, and much more.
Everyday Uses for a Date Calculator
- Counting days until a vacation, wedding, or deadline
- Calculating how many days you have been alive (age in days)
- Planning a pregnancy timeline or due date countdown
- Figuring out billing cycles, payment due dates, or rent periods
- Project management: calculating durations and milestones
- Historical research: finding the gap between two events
- Counting business days for legal or shipping purposes
How Days Between Dates Are Calculated
Calculating the days between two dates might sound simple, but leap years and varying month lengths make it trickier than just counting on your fingers. Here is how it works:
- Convert both dates to a common reference point (usually days since a fixed date like January 1, year 0, or the Julian Day Number system).
- Account for leap years — years divisible by 4 are leap years, except century years not divisible by 400. (2000 was a leap year; 1900 was not; 2100 will not be.)
- Subtract the earlier date from the later date to get the total number of days.
- Convert total days into years, months, and remaining days by working backward through the calendar.
Date Calculation Example
How many days are between March 15, 2024, and September 10, 2024?
- March 15 to March 31: 16 days
- April: 30 days
- May: 31 days
- June: 30 days
- July: 31 days
- August: 31 days
- September 1 to 10: 10 days
- Total: 179 days
In years and months, that is about 5 months and 26 days. Our calculator does this all instantly for any valid dates.
Business Days (Weekdays)
For many professional purposes, you want to count only business days (Monday through Friday), excluding weekends. This is especially important for:
- Shipping and delivery estimates
- Contract deadlines and notice periods
- Payroll and pay period calculations
- Project timelines and task estimates
Our calculator includes a business days count. Keep in mind that it does not subtract public holidays, which vary by country and region. For precise legal or business purposes, always check whether holidays need to be excluded as well.
Adding and Subtracting Dates
Sometimes you need to know what date it will be a certain number of days, weeks, months, or years from now. For example:
- “What day is 90 days from today?”
- “My license expires in 18 months — when is that?”
- “What was the date 30 days ago?”
- “My baby is due in 280 days — what is the due date?”
Use the “Add/Subtract Days” mode in the calculator to answer these questions instantly. You can combine years, months, weeks, and days all at once.
Frequently Asked Questions
How do I calculate days between two dates in Excel?
In Excel or Google Sheets, use the DATEDIF function or simply subtract one date from another (=B1-A1) and format the result as a number. For business days only, use the NETWORKDAYS function.
What is the Julian calendar vs. the Gregorian calendar?
Most of the world uses the Gregorian calendar (introduced in 1582), which corrected the Julian calendar's slight drift by removing 10 days and adjusting leap year rules. Our calculator uses the Gregorian system, which is the standard for everyday use.
How do leap years work?
A leap year has 366 days (with February 29 added) instead of the usual 365. The rules are:
- A year divisible by 4 is a leap year
- EXCEPT century years (ending in 00), which are NOT leap years
- UNLESS the century year is divisible by 400 — then it IS a leap year
So 2020, 2024, and 2000 were leap years, but 1900 and 2100 are not.