Date Calculator
Calculate the number of days between two dates, or add/subtract days from a date to find a target date.
How to Use
- Select a calculation mode: "Date Difference" or "Add/Subtract Days".
- Date Difference: Enter start and end dates, and the system automatically calculates the interval in years, months, days, and total days.
- Add/Subtract Days: Enter a base date and number of days (positive for forward, negative for backward), and the system displays the resulting date and day of the week.
- Results are displayed in real-time — no need to click a button.
Calculation Examples
Date Difference Example
Start: January 1, 2025. End: December 31, 2025
Result: 11 months 30 days apart (364 days total)
Add/Subtract Days Example
Base date: March 1, 2025, plus 90 days
Result: 2025/5/30 (Friday)
Practical Use Cases
- Calculate a due date (last menstrual period + 280 days)
- Count down days until retirement
- Calculate contract expiration or probation end date
- Calculate how many days a trip spans
Important Notes
- Leap year handling:This calculator automatically handles leap years (February 29) — no special action needed.
- Year/month/day calculation method:The "Date Difference" mode calculates by calendar (e.g., January 31 to March 1 = 1 month and 1 day), not by dividing by a fixed 30 days per month.
- Total days calculation:Total days is the exact number of days between the two dates, not including the start date itself (i.e., end date minus start date).
- Negative days:The Add/Subtract Days mode supports negative numbers, which count backward from the base date.
- This calculator is for reference only. Legal or business day calculations may follow different counting rules.
Frequently Asked Questions
How do I calculate the number of days between two dates?
Select "Date Difference" mode, enter the start and end dates, and the system will automatically calculate the interval in years, months, days, and total days. The result is calculated as "end date minus start date", not including the start date itself.
What is a leap year? Which years are leap years?
Leap year rules: (1) Years divisible by 4 are leap years; (2) Years divisible by 100 are not leap years; (3) Years divisible by 400 are leap years. For example, 2024 is a leap year (divisible by 4), 1900 is not (divisible by 100 but not 400), and 2000 is (divisible by 400). Leap years have 29 days in February.
Can I subtract days in the "Add/Subtract Days" mode?
Yes. Enter a negative number in the days field to calculate a date in the past. For example, with a base date of March 15, 2025 and -30 days, the result is February 13, 2025.
How many days are in a year? What is the average number of days in a month?
A common year has 365 days, and a leap year has 366. The average is about 365.25 days per year. Monthly days vary: January, March, May, July, August, October, and December have 31 days; April, June, September, and November have 30; February has 28 (29 in leap years). The average month is about 30.44 days.
How do I calculate a pregnancy due date?
The general method is "first day of last menstrual period + 280 days (40 weeks)". Use the "Add/Subtract Days" mode, set the base date to the last menstrual period date, and enter 280 days to get the estimated due date. Note that actual delivery dates vary — always follow your doctor's professional guidance.