How do I add days to a date?

Enter your start date in the 'Add/Subtract Duration' tab, set the number of days (or weeks, months, years), choose Add or Subtract, and click Calculate. The result date is shown instantly with the day of the week.

How are business days calculated?

Business days exclude Saturdays and Sundays. The calculator counts only weekdays (Monday through Friday) between the two dates. It does not account for public holidays, as these vary by country and region.

How many days are in a year?

A regular year has 365 days. A leap year has 366 days. Leap years occur every 4 years, except for years divisible by 100, which are not leap years unless also divisible by 400. So 2000 was a leap year, but 1900 was not.

What is the difference between calendar months and calendar days?

Calendar days count every day including weekends. Calendar months vary in length (28–31 days). When adding months to a date, if the resulting day doesn't exist in that month (e.g., March 31 + 1 month), it's clamped to the last day of that month (April 30).

How do I find what day of the week a date falls on?

Enter the date in any input field and the day of the week is shown automatically. You can also use the Weekday Finder tab to look up any historical or future date.

Date Calculator

Add/subtract durations from a date, find differences, calculate business days, and find weekdays.

Start Date

Operation

Years

Months

Weeks

Days

Start Date

End Date

Start Date

End Date

Date

Choose a mode and enter dates to calculate.

How to Use the Date Calculator

Add / Subtract: Enter a start date, choose to add or subtract, fill in years/months/weeks/days, then click Calculate. Perfect for "what date is 90 days from today?"
Date Difference: Enter two dates to see how many days, weeks, months, and years are between them. Great for age calculations and project timelines.
Business Days: Enter two dates to count only working days (Monday–Friday), excluding weekends. Useful for SLA calculations and project planning.
Weekday Finder: Enter any date to instantly see what day of the week it falls on — past or future.

Date Arithmetic Explained

Date arithmetic is more complex than simple number addition because months have different lengths, leap years add an extra day, and time zones can shift dates. This calculator works entirely in local dates (no time component) to keep results intuitive. When adding months, the result is clamped to the last valid day of the month — for example, January 31 + 1 month = February 28 (or 29 in a leap year), since February 31 does not exist.

Leap Years

A leap year has 366 days, occurring when a year is divisible by 4, except for century years (divisible by 100) which must also be divisible by 400. So 2000 was a leap year (divisible by 400), 1900 was not (divisible by 100 but not 400), and 2024 is a leap year (divisible by 4). This means adding "1 year" to February 29 in a leap year results in February 28 in the following non-leap year.

Business Days vs. Calendar Days

Business days (also called working days) count only Monday through Friday, excluding weekends. This calculator does not deduct public holidays, as these vary significantly by country, region, industry, and company policy. For contract deadlines, SLA (Service Level Agreement) calculations, and legal notices, always confirm whether "business days" in your jurisdiction includes or excludes specific public holidays. Many US contracts use "business days" to mean weekdays excluding federal holidays.

Common Use Cases

The date calculator is useful for: calculating project deadlines (add 90 calendar days to contract signing date), determining age in days or years, finding the number of working days in a quarter, checking whether a 30-day notice period has passed, planning subscription renewals and billing cycles, scheduling recurring events, and verifying ISO week numbers for payroll processing. Developers use it to test edge cases around month boundaries, DST transitions, and year rollovers.

ISO Week Numbers

The ISO 8601 week numbering system defines week 1 as the week containing the first Thursday of the year. ISO weeks always run Monday through Sunday. A year has either 52 or 53 ISO weeks. This means January 1 can be in week 52 or 53 of the previous year. ISO week numbers are widely used in European manufacturing and payroll systems, and in programming (Python's datetime.isocalendar(), JavaScript's getISOWeek polyfills).

Frequently Asked Questions

Use the "Add / Subtract" tab. Enter your start date, set the number of days (and optionally weeks, months, or years), choose "Add", then click Calculate. The result date and day of the week appear instantly.

Business days count only weekdays (Monday–Friday), skipping Saturdays and Sundays. Public holidays are not excluded because they vary by country. For SLA and contract work, verify your jurisdiction's definition of "business day."

A regular year has 365 days. A leap year has 366 days. Leap years occur every 4 years, except century years not divisible by 400. The next leap years are 2028, 2032, 2036, 2040.

Calendar days count every day (including weekends). Calendar months vary in length (28–31 days). Adding 1 month to January 31 gives February 28/29 (clamped to the last valid day). Adding 30 days always adds exactly 30 calendar days.

Use the "Weekday Finder" tab. Enter any date — past or future — and the day of the week is shown instantly. You can also see the day in the Add/Subtract and Difference results.