Date calculator
Count the days between 1 and 31 January 2026, or add a number of days such as 7 to a start date.
- Runs in your browser
- No upload
- Instant result
Assumptions & formula
Formula and assumptions
Δd = (t₂−t₁)/86400000; d₂=d₁+n
Results
Enter values and press Calculate to see the result here.
How to use this calculator
- Set inputs and units
Choose the difference in calendar days, or adding signed days. Enter ISO dates, not times.
- Calculate
Check this worked example: 2024-02-28 → 2024-03-01: 2 days; 2024-02-28 +1 day → 2024-02-29
- Results
The difference is the end date minus the start date. It doesn't count both endpoint dates as separate inclusive days.
Questions and limits
Does daylight saving change a calendar-day result?
No. Dates use UTC calendar arithmetic, not the elapsed local hours between timestamps.
Which input errors should I correct?
Use real YYYY-MM-DD dates and whole signed day counts. Impossible dates are rejected, not rolled forward.
What does this calculation leave out?
No workday, holiday, legal-deadline or appointment-scheduling rules are assumed.