This date calculator adds or subtracts 100 days (or any day count) from a date and counts days between two dates using UTC-stable arithmetic.
Everyday life
100 Day Calculator
Add, subtract, or count calendar days using UTC-stable date arithmetic.
Calculator
About this calculator
Formula notes
result = start date + days x 86400000 ms; inclusive counting adds 1 when the start date is counted.
Worked examples
- Example: 100 days after 2026-01-01, excluding the start date, lands on 2026-04-11.
- 100 days before 2026-01-01, including the start date, lands on 2026-09-24.
How to use it
- Choose add days, subtract days, or days between.
- Enter the start date and the day count or end date.
- Choose whether to include the start date, then read the result.
Frequently asked questions
Is the start date included by default?
No. The default counting excludes the start date, matching a counter that starts from the next day.
Does time-zone affect the result?
No. Dates are evaluated as UTC calendar days.
Limits and interpretation
- Dates use UTC calendar days, so results are stable across time zones but ignore local-time conventions.
- The inclusive option shifts the result by one day; confirm whether your workflow counts the start date.