How Date Calculations Work
A date calculation converts two calendar dates into a measurable duration. Subtract the earlier date from the later one, and you get the total number of calendar days. Divide by 7 for weeks. Compare year and month components for calendar months and years. Every result in this calculator uses the actual Gregorian calendar, so leap years, varying month lengths, and real weekday positions are all accounted for.
The “add days” and “subtract days” modes work in reverse: start from a known date, shift forward or backward by N days, and land on the resulting date. The calculator shows the day of the week for the result, plus a breakdown of how many weekdays and weekend days fall within that span.
Calendar Days vs Business Days
Calendar days count every day — Monday through Sunday, holidays included. Business days (also called working days or weekdays) count only Monday through Friday. A 30-calendar-day period typically contains about 22 business days and 8 weekend days, though the exact split depends on which day of the week you start.
This distinction matters for contracts, shipping estimates, legal deadlines, and project planning. “Net 30” payment terms mean 30 calendar days. A “5 business day” delivery window means one calendar week. This calculator shows both counts so you can use whichever applies to your situation.
Common Date Spans at a Glance
| Period | Calendar Days | ~Weekdays | ~Weekends |
|---|---|---|---|
| 30 days | 30 | ~22 | ~8 |
| 60 days | 60 | ~43 | ~17 |
| 90 days | 90 | ~64 | ~26 |
| 6 months (182 days) | 182 | ~130 | ~52 |
| 1 year (365 days) | 365 | ~261 | ~104 |
| 1 leap year (366 days) | 366 | ~262 | ~104 |
These are approximations. The exact weekday/weekend split depends on the start day. Use the calculator above for precise counts for your specific dates.
Leap Years Explained
A leap year adds one extra day — February 29 — making the year 366 days instead of 365. Leap years occur every 4 years, with exceptions: years divisible by 100 are not leap years unless also divisible by 400. So 2000 was a leap year, 1900 was not, and 2024 was a leap year.
This affects date math when your range crosses a February in a leap year. A span from February 1 to March 1 is 28 days in a normal year but 29 days in a leap year. The calculator handles this automatically — you do not need to adjust anything.
When You Need Date Math
- Project deadlines: “The client gave us 90 days. When is that?”
- Lease terms: “My 12-month lease started March 15. When does it end?”
- Event countdowns: “How many days until my wedding on September 20?”
- Shipping estimates: “7–10 business days from today. What dates are those?”
- Contract durations: “The contract runs January 1 to June 30. How many weekdays is that?”
- Legal deadlines: “The statute of limitations is 2 years from the incident date.”
The Formula
Days Between = End Date − Start Date (in milliseconds) ÷ 86,400,000
Under the hood, JavaScript converts dates to Unix timestamps (milliseconds since January 1, 1970), subtracts them, then divides by the number of milliseconds in one day (86,400,000). This gives an exact integer number of calendar days. Weeks, months, and years are derived from this base count plus calendar-aware logic for month boundaries.
How to Calculate Your Age in Days
Set the start date to your birthday and the end date to today. The calculator returns your exact age in days, weeks, months, and years, plus how many of those days were weekdays vs weekends. For more detailed age breakdowns including hours and minutes, use the age calculator.
Need to calculate percentages? Try the percentage calculator. For age-specific breakdowns with years, months, and days of life, use the age calculator.