Add Days to Date Calculator

How to Add Days to a Date — Complete Guide

Date arithmetic is one of the most common calculations in everyday life, yet it trips people up constantly. Adding "30 days" to the 15th of March gives the 14th of April — not the 15th. Adding "one month" to the 31st of January produces a different result depending on whether the following month is February (28 or 29 days) or March (31 days). This guide explains every scenario.

Adding Calendar Days

Adding calendar days is the simplest operation: count forward (or backward) the specified number of days, treating every day equally. The calculator crosses month and year boundaries automatically.

Adding Weeks

One week is always exactly 7 days. Adding 4 weeks to 1 May 2025 gives 29 May 2025 — not 1 June. This distinction matters in employment law, where statutory notice is measured in weeks (7-day units), not calendar months.

Adding Months — End-of-Month Rules

When adding months, what happens when the resulting month is shorter than the start month? The ISO 8601 standard, and most date libraries, clamp to the last day of the shorter month:

Adding Working Days Only

When your calculation must exclude weekends and/or bank holidays, tick the "Working days only" option. The calculator iterates through each day, skipping Saturdays, Sundays, and any UK bank holidays in the list.

Common Use Cases

Contract Deadlines (30 Days)

Many commercial contracts give counterparties 30 days to respond, cure a breach, or make a payment. This is 30 calendar days from the date of notice. If you send a notice on 15 March 2025, the deadline falls on 14 April 2025. Missing it by even one day may have serious contractual consequences.

Notice Periods

UK employment law sets a statutory minimum notice period of one week per complete year of service, up to 12 weeks for 12 or more years of service. Your contract may specify a longer period — typically 1 month, 3 months, or 6 months for senior roles. Enter the date notice was given and your notice period in days or weeks to find your last working day.

Warranty Expiry

Consumer electronics typically carry a 1-year manufacturer's warranty (12 months from purchase date) plus, in the UK, rights under the Consumer Rights Act 2015 for 6 years. Enter your purchase date and add 12 months to find when the manufacturer warranty expires, then add 6 years to find the Consumer Rights Act limit.

Subscription Renewal

Monthly subscriptions renew exactly one calendar month from the sign-up date. A subscription started 31 January renews on 28 February (or 29 in a leap year), then 28 March, then 28 April — because the clamp applies each month independently from the preceding renewal date, not from the original date.

Tax Deadlines

HMRC Self Assessment online filing is due 31 January. Payment on account deadlines are 31 January and 31 July. Corporation Tax is due 9 months and 1 day after the end of the accounting period. Add the relevant period to your period-end date to confirm the exact payment deadline.

Pregnancy Milestones

A standard pregnancy is divided into three trimesters of approximately 13 weeks each. From the first day of the last menstrual period (LMP): add 13 weeks for end of first trimester, 26 weeks for end of second trimester, and 40 weeks for the estimated due date. The NHS uses 280 days (40 weeks) from LMP as the standard gestation period.

ISO 8601 Date Format

Internally, our calculator works with dates in ISO 8601 format (YYYY-MM-DD), which is the internationally recognised standard for unambiguous date representation. When you enter 15/03/2025 in the UK format, the calculator converts it to 2025-03-15 before performing arithmetic. Results are displayed back in DD/MM/YYYY UK format.

What Was 90 Days Ago?

Enter today's date, type 90, select "Days", and choose "Subtract". From 20 February 2026, subtracting 90 days gives 23 November 2025. This is useful for legal limitation periods, reviewing invoices, or checking eligibility windows.