Date Calculator
Find the exact duration between two dates, or add or subtract days, weeks, months, or years from any date.
How date math works
A "day" is one rotation of the Earth — 86,400 seconds. A "year" is approximately 365.25 days, which is why we have leap years every 4 years (with century exceptions). A "month" is even less precise, ranging from 28 to 31 days. Date math gets tricky because of these irregular lengths, which is why a calculator beats mental arithmetic.
Add or subtract: handling month overflow
Adding months rolls over automatically. If you add 1 month to January 31, you get February 28 (or 29 in a leap year) because February has no 31st. The JavaScript engine and most calendar apps apply this rule. Adding 1 year to February 29 gives February 28 in non-leap years, then back to February 29 every 4 years.
Common uses
- • Project planning — count days remaining until launch, deadline, or event
- • Tenancy and contracts — calculate notice periods, lease end dates, or rental terms
- • Age in days — sometimes required for visa applications, age-restricted products, or milestones
- • Pregnancy and parenting — due-date countdown or age tracking in weeks and months
- • Billing and finance — invoice due dates, interest accrual periods, EMI scheduling
Frequently asked questions
How do I calculate the number of days between two dates?
Pick a start date and end date and the calculator instantly shows the duration in years, months, days, weeks, hours, minutes, and seconds. The default mode is exclusive (does not count the end date). Toggle "Include end date" to make it inclusive.
Does the calculator account for leap years?
Yes. All calculations use the actual Gregorian calendar with proper handling of leap years. The difference between Jan 1, 2024 and Jan 1, 2025 is 366 days, not 365, because 2024 is a leap year.
How do I add 30, 60, or 90 days to a date?
Use the "Add or subtract" tab. Pick your base date, choose Add, enter the number of days, and the calculator returns the resulting date. Month and year boundaries are handled automatically.
Can I calculate my age in days?
Yes. Set your date of birth as the start date and today as the end date. The result shows total days you've been alive, plus a breakdown in years, months, and days.
Is the date calculator accurate for historical dates?
Yes. The calculator works for any date in the proleptic Gregorian calendar. It is accurate for historical dates, future dates, and dates spanning the year 2000 leap-year boundary.