Add or subtract days from any date

How date addition works

Years and months are added first, then weeks and days. Adding months respects the length of the resulting month. For example, adding 1 month to January 31 gives February 28, or February 29 in a leap year, not March 3.

This is useful for calculating deadlines, expiry dates, contract end dates, and countdowns for any duration.

Quick reference

Month-end edge cases

Month addition is the trickiest part of date arithmetic because months have different lengths. The standard rule used by this calculator — and by most programming languages and legal conventions — is to land on the last valid day of the target month when the source day does not exist there.

Examples:

• January 31 + 1 month = February 28 (or 29 in a leap year)
• January 31 + 2 months = March 31
• August 31 + 1 month = September 30
• October 31 + 4 months = February 28 (or 29 in a leap year)

This is called "end-of-month clamping." It avoids spilling into the next month (March 3 instead of February 28), which would change the month count and violate the intent of "+1 month."

Calendar days vs business days

Adding calendar days is straightforward: 30 days from any date is exactly 30 days later, regardless of weekends or public holidays. Adding business days is more complex and depends on which public holidays are observed in the relevant jurisdiction — these vary by country, state, and even industry.

For most legal and contractual purposes, "days" means calendar days unless explicitly stated otherwise. "Business days" or "working days" means days excluding weekends and named public holidays. The calculator adds calendar days; if you need business days, add them manually or account for weekends in your count (roughly 20–22 business days per 30 calendar days).

Practical applications

Contracts and legal deadlines

Contract durations are almost always expressed in months or years: a 12-month lease, a 3-year service agreement, a 90-day right of withdrawal. The end date needs to be calculated precisely, because "one year from March 15" is March 15 of the following year, not 365 days later (which would be March 14 in a non-leap year, or March 15 in a leap year).

Trial periods and subscriptions

Free trials, return windows, and subscription billing cycles are typically expressed in days: 14-day free trial, 30-day return policy, 60-day money-back guarantee. These are calendar day counts from the start date. Adding the exact number of days gives the precise expiry date.

Probation and notice periods

Employment probation periods are usually stated in months (3-month probation, 6-month probation). Notice periods are stated in weeks or months. Adding these to a start date gives the exact end date of the probation or the earliest departure date after serving notice.

Project planning and scheduling

Adding durations to milestone dates gives a forward-looking project schedule. Working backwards from a fixed deadline — subtracting durations — shows the latest possible start date for each phase. Both directions use the same date arithmetic.

Warranty and guarantee expiry

Product warranties, insurance policies, and guarantee periods are all date-bounded. A 2-year warranty starting on a purchase date expires exactly 2 years later, which is not always 730 or 731 days later due to leap years — it is the same day and month two years hence.

How to calculate a date manually

For day addition: convert to a day number, add, convert back. For month addition: add the month count to the month number, carry over to years if needed, clamp the day to the last day of the resulting month. For year addition: add directly to the year, then apply the same leap-year clamp if the original date was February 29.

The calculator handles all of this automatically — including leap year detection and month-end clamping — so you can enter any start date and any combination of years, months, weeks, and days and get a reliable result.