Date Add/Subtract Guide: Add Days, Weeks, Months, and Years (Without Surprises)

Adding days and weeks

Adding days/weeks is usually straightforward: you move forward or backward by a whole number of days.

When working with date-only values (YYYY-MM-DD), adding N days means stepping N calendar days forward (or backward if negative).

This is the most consistent operation and is commonly used for deadlines, reminders, prescriptions, and countdowns.

Weeks are simply 7 days — adding 2 weeks is exactly the same as adding 14 days.

Adding months/years and end-of-month rules

Months and years have variable lengths. If the target month has fewer days, calculators typically clamp to the last valid day.

Example: adding 1 month to Jan 31 is tricky because February often doesn't have a 31st. Most tools return Feb 28/29 (end-of-month clamping).

This also affects leap years: adding 1 year to Feb 29, 2024 gives Feb 28, 2025 since 2025 isn't a leap year.

If your use-case requires a different rule (e.g., "same day-of-month or skip"), you'll need custom logic or to use days instead.

Common use cases

Date arithmetic is used everywhere — from project management to healthcare to legal deadlines.

**Project deadlines:** Add days or weeks to estimate delivery dates. "Ship in 2 weeks" = today + 14 days.

**Prescription refills:** Know when your 30-day or 90-day supply runs out. Add the days to your last refill date.

**Contract renewals:** Add 1 year (or 6 months) to a contract start date to plan renewals.

**Visa expirations:** Many visas are valid for 90 days or 6 months from entry. Add those days to your entry date.

**Legal deadlines:** Court filings often have deadlines like "30 days from service date" or "1 year from incident."

Subtracting and reversing the operation

Subtracting is just adding a negative number, but reversing can be non-trivial for months/years when clamping occurred.

For days, subtraction is perfectly reversible: if you add 10 days and then subtract 10 days, you get back the original date.

For months/years, clamping can break reversibility: Jan 31 + 1 month → Feb 28; then Feb 28 - 1 month → Jan 28 (not Jan 31).

This asymmetry is important in financial/legal contexts where the original date matters.

Tips for accurate date math

Follow these best practices to avoid common pitfalls in date calculations.

**Use days for exact intervals:** If you need exactly 30 days or 365 days, use days — not months or years.

**Be aware of DST:** This calculator uses date-only math (no time), so DST doesn't affect results. But if you're working with datetime values elsewhere, DST can cause surprises.

**Document the rule:** When building systems, document whether you use "same day of month," "end of month," or "clamp" rules.

**Test edge cases:** Always test with dates like Jan 31, Feb 28/29, and Dec 31 to catch clamping issues.