Add / Subtract Days Calculator

Date Arithmetic — Adding and Subtracting Time

Adding or subtracting days from a date is essential for calculating deadlines, due dates, payment schedules, subscription renewals, and project milestones. This calculator handles days, weeks, months, and years — and shows the day of the week of the result.

Month and year arithmetic is more complex than day arithmetic because months have different lengths. Adding 1 month to January 31 gives February 28/29 (last day of February), not March 3. Our calculator handles these edge cases correctly.

How Date Arithmetic Works

Add N days: Result = Start + N × 86,400,000 ms Add N months: Advance month by N, clamp to last day if needed Add N years: Advance year by N, handle Feb 29 in non-leap years

Common Date Calculations

Calculation	Use Case
Today + 30 days	Net-30 payment terms
Today + 90 days	Quarterly deadlines
Date + 14 days	Two-week notice period
Date + 1 month	Monthly subscription renewal
Date − 2 weeks	Prep deadline before event

Frequently Asked Questions

Enter today's date and add 30 days. Net-30 payment terms mean payment is due 30 calendar days after the invoice date. Similarly, Net-60 is 60 days and Net-90 is 90 days. These are common B2B payment terms.

Enter your project start date, select 'Add', enter the number of days (or weeks/months), and the calculator shows the exact deadline date. Use the weekday display to avoid scheduling deadlines on weekends or adjust accordingly.

When adding months to a date that falls at the end of a month, the result is clamped to the last valid day. Adding 1 month to January 31 gives February 28 (or 29 in a leap year), not March 3. This is the standard behavior followed by most date libraries and calendar systems.

Enter your birthdate in the Start Date field, select 'Subtract', and enter today's date using the date difference calculator. Or use our dedicated Age Calculator, which shows your age in days, hours, and minutes automatically.