Date Calculator: How to Calculate Days Between Dates, Add or Subtract Days and Never Miss a Deadline Again

A few months ago, a friend asked me: "My tenancy agreement says I need to give 60 days' notice. I want to leave on 1 August. When do I need to send the notice?" She'd been counting on her fingers, getting confused by months with different lengths, and wasn't sure whether to count the start date or the end date.

The answer took our free date calculator about one second: 2 June. But getting it wrong could have cost her a month's rent.

Date calculations seem simple until you actually try to do them. Months have different lengths, leap years throw things off, and the difference between "calendar days" and "working days" catches people out constantly. This guide covers everything you need to know about calculating dates — and when it really matters to get it right.

Three Types of Date Calculation

1. Days Between Two Dates

The most common calculation. You have two dates and need the gap between them:

• How many days until my holiday?
• How long have I been in my current job?
• How many days between order and delivery?
• How many days since a specific event?

2. Add Days to a Date

Start with a date and add a number of days, weeks, or months forward:

• What date is 90 days from today?
• When does a 30-day warranty expire?
• What is the date 6 months from now?
• When is 14 days after my positive COVID test?

3. Subtract Days from a Date

Work backwards from a deadline:

• When should I start a 12-week project to finish by 1 September?
• What date was 60 days ago?
• When is 28 days before my court hearing?
• When should I give notice to leave by a specific date?

Our date calculator handles all three types instantly.

Where Date Calculations Matter Most

Legal Notice Periods

UK law is full of specific time periods that must be calculated precisely:

Getting these wrong has real consequences — missed deadlines can mean lost deposits, legal penalties, or forfeited rights.

Financial Deadlines

Money and dates are inseparable:

• Tax return deadlines — 31 January for online self-assessment, 31 October for paper
• Payment terms — Net 30, Net 60, Net 90 (see our invoice generator for more on payment terms)
• Credit card billing cycles — typically 25-30 days
• Mortgage payment dates — use our mortgage calculator to plan repayments
• ISA deadlines — 5 April each year (end of tax year)
• Savings interest periods — check with our savings calculator

Project Management

Every project has milestones and deadlines. Calculating realistic timelines requires understanding:

• How many working days between start and deadline
• Bank holidays that fall within the project period
• Buffer time for reviews and revisions
• Dependencies between tasks

Pregnancy and Medical

Pregnancy is measured in weeks from the first day of the last menstrual period (LMP). Medical follow-ups are often scheduled in specific day intervals. Our pregnancy calculator handles pregnancy-specific date calculations, including trimester dates and due date estimation.

Calendar Days vs Working Days

This distinction trips people up constantly:

When a contract says "within 10 days," always check whether it means calendar or working days. The difference is significant — 10 working days is actually 2 full weeks.

UK Bank Holidays 2025

Scotland and Northern Ireland have additional bank holidays. Always check which apply to your situation.

The Month Length Problem

Months have different lengths, which makes "one month from today" surprisingly ambiguous:

• January: 31 days
• February: 28 days (29 in leap years)
• March: 31 days
• April: 30 days
• May: 31 days
• June: 30 days
• July: 31 days
• August: 31 days
• September: 30 days
• October: 31 days
• November: 30 days
• December: 31 days

If today is 31 January and you add "one month," is the result 28 February or 3 March? Different systems handle this differently. Our date calculator uses the most common convention: it caps at the last day of the target month (so 31 January + 1 month = 28 February).

Useful Date Formulas

Common Date Calculations People Search For

• How many days until Christmas?
• What date is 90 days from today?
• How many weeks between two dates?
• What day of the week was I born?
• How many working days in a month?
• When is exactly 6 months from now?
• How many days until my birthday?
• What date is 30 days from today?
• How many days in a year?
• When is the next bank holiday?

Our date calculator answers all of these instantly.

Date Calculations for Specific Situations

Pregnancy Due Date

Estimated due date is 280 days (40 weeks) from the first day of your last menstrual period. Use our dedicated pregnancy calculator for detailed week-by-week tracking.

Passport Renewal

UK passports should be renewed before they expire. If you're travelling, many countries require at least 6 months' validity remaining. Calculate 6 months before your travel date to know your deadline.

Probation Periods

Most UK employment contracts include a 3 or 6-month probation period. Calculate the exact end date from your start date to know when your full employment rights kick in.

Other Useful Date and Time Tools

• Age Calculator — calculate exact age from date of birth
• Pregnancy Calculator — due date and week-by-week tracking
• Exam Timer — countdown timer for timed tasks
• Loan Calculator — calculate repayment schedules with dates

Try Our Free Date Calculator

Whether you're calculating notice periods, planning project deadlines, counting down to an event, or working out financial dates, our free date calculator gives you instant, accurate results. Days between dates, add days, subtract days — all completely free with no sign-up required.

Frequently Asked Questions

How many days are between two dates?

Enter both dates into our date calculator and get the exact number of days, weeks, and months between them instantly.

What date is 90 days from today?

Use the "add days" feature in our calculator. Enter today's date, add 90 days, and get the exact result — accounting for different month lengths and leap years.

How many working days are in a month?

Typically 20-23 working days, depending on the month and bank holidays. February has the fewest (usually 20), while months with no bank holidays have 22-23.

Does the calculator account for leap years?

Yes. Our date calculator correctly handles leap years, including the century rule (years divisible by 100 are NOT leap years unless also divisible by 400).

What's the difference between calendar days and working days?

Calendar days include every day (weekends and holidays). Working days are Monday to Friday, excluding public holidays. 10 working days equals approximately 14 calendar days.