Date Calculator — Add or Subtract Days
Our free date calculator tells you exactly what date you get when you add or subtract days, weeks, months, or years from any start date — plus the weekday, the total days between the dates, and a weeks-and-days breakdown. Perfect for deadlines, due dates, countdowns, and lookbacks.
Enter your numbers
Your results
| Days between start and result | — |
| ISO date | — |
Day counts use calendar days (weekends included), counting forward from the start date — so +1 day lands on the next calendar date.
How to use this calculator
Pick a start date (today is pre-filled), enter how many days, weeks, months, or years to move, and choose Add or Subtract. The resulting date, its weekday, and the day count update instantly as you type.
Use Add for forward dates — project deadlines, delivery windows, "90 days from today" questions — and Subtract to look back, e.g. finding the start date of a "last 30 days" period.
How it works
Days and weeks shift the calendar by whole days, so the math is exact: +90 days always spans exactly 90 calendar days.
Months and years are trickier, because months aren't all the same length. Plain JavaScript date math has a known quirk: adding 1 month to January 31 rolls over into March (March 3), silently skipping February. This calculator avoids that by clamping to the last day of the target month: January 31 + 1 month = February 28 (or February 29 in a leap year), May 31 + 1 month = June 30, and February 29, 2024 + 1 year = February 28, 2025. The day-of-month is preserved whenever the target month has enough days.
Frequently asked questions
What is 90 days from today?
Enter 90 in the number field, choose days, and make sure Add is selected — the result updates instantly below. Counting manually is error-prone because months have different lengths, so a date calculator is the reliable way to get the exact date.
How are months added when the start day doesn't exist in the target month?
This calculator clamps to the last day of the target month. For example, January 31 + 1 month = February 28 (or February 29 in a leap year), and May 31 + 1 month = June 30. This avoids the "rollover" quirk of plain date math, which would push the result into the next month.
How does the calculator handle leap years?
Leap days are counted automatically for day-based calculations. For month and year math, February 29 exists only in leap years: adding 1 year to February 29, 2024 gives February 28, 2025, and January 31 + 1 month in a leap year gives February 29.
What's the difference between adding and subtracting dates?
Adding moves the result forward in time from the start date (useful for deadlines, due dates, and countdowns), while subtracting moves it backward (useful for finding start dates or looking back a period, like "90 days ago"). Both use the same calendar-day, week, month, and year math — only the direction changes.
Can this calculator count business days?
No — this tool counts calendar days, weekends included. If you need the day count between two specific dates, see our Days Between Two Dates Calculator.