Date Calculator
Add or subtract days, weeks, months, and years from a date, or calculate the duration between two dates.
Start from a date and add or subtract a period of time. Use negative numbers to subtract.
Calculate the number of days between two dates. The start date is excluded from the count.
How to use this tool
- Choose between Add / Subtract mode or Days Between mode.
- For Add / Subtract: enter a start date and the number of years, months, weeks, and days to add. Use negative numbers to subtract.
- For Days Between: enter two dates to find how many days separate them.
- Click Calculate to see the result.
The calculator handles leap years and month boundaries correctly. For example, adding one month to January 31 gives February 28 (or 29 in a leap year), since February doesn't have 31 days.
Date Arithmetic, Epochs, and Scheduling Applications
Date calculations are vital for contract deadlines, legal notice periods, project milestones, warranty tracking, and event planning. Because our calendar is divided into irregular months and leap cycles, manual date math frequently causes scheduling errors:
| Operation | Algorithm | Use Case |
|---|---|---|
| Add/Subtract Days | Timestamp ± (N × 86,400,000 ms) |
Finding a 30-day, 60-day, or 90-day invoice due date |
| Add/Subtract Months | Month rollover + clamp to month-end |
Calculating pregnancy trimesters or rental leases |
| Date Difference | |End Date − Start Date| in days |
Days remaining until a holiday, wedding, or launch |
Standard Julian vs Gregorian Day Count
Modern date arithmetic follows the Gregorian calendar introduced in 1582. The tool standardizes leap cycles, month boundaries, and year-ends so you obtain verifiable calendar calculations every time.
Frequently asked questions
Select the 'Duration Between Dates' tab, choose your start date and end date, and click Calculate. The tool computes the total days, full weeks plus remaining days, and elapsed calendar months.
When adding a month to dates ending on the 31st (for example, January 31), the target month may have only 28, 29, or 30 days. The calculator automatically adjusts to the last valid day of that month (e.g., January 31 + 1 month = February 28 or 29 in a leap year).
This tool calculates total calendar days (all 7 days per week). To approximate business days, multiply the total full weeks by 5 and add remaining weekdays.
No. BrightKit measures calendar dates using midnight-to-midnight UTC date boundaries, eliminating the 23-hour or 25-hour anomalies caused by daylight saving transitions in spring and autumn.
Yes. You can subtract any combination of days, weeks, months, or years from a reference date, or calculate the duration between historical dates going back centuries.