A walkthrough, end to end.
- 1
Pick a start date and an operation (add or subtract).
- 2
Enter the number of days, weeks, months, or years.
- 3
See the resulting date and the day of the week.
Date arithmetic
JavaScript's Date object handles month rollover and leap years. Months and years use calendar arithmetic; days and weeks use exact 24-hour blocks.
What you can do with this.
Project deadlines
Add 30 days to a kickoff to find the milestone date.
Visa expirations
Compute the exact exit date from a 90-day window.
Pregnancy estimates
Add 280 days to LMP for a due date.
Subscription renewals
Compute the next billing date.
Probation dates
Add 6 months from start date for the review date.
Lease end dates
Add 12 months to the move-in date.
Birthday math
Find the date 100 days before someone turns 30.
Date 2026 — what's current
Standard date math; calculator wins for instant calendar correctness.
Frequently asked.
Adding 1 month to Jan 31 gives Feb 28 (or 29 in leap years). Standard calendar carry.
Use the subtract operation for past dates.
No. All math runs locally in your browser.
Calculations are date-level; time-of-day is treated as midnight.