Date Calculator
Calculate days between dates, add or subtract durations, and find exact age from a birthday — with business days and full breakdowns.
Updated
What is the Date Calculator?
A date calculator that handles everything in one place: find the exact number of days between two dates, add or subtract any duration from a date, and calculate a precise age from a birthday. Unlike most online tools that force you to jump between separate pages, this calculator covers all three use cases with instant, live results — no "Calculate" button required.
Every result goes beyond a raw number. The Date Difference mode shows total days alongside business days (Monday–Friday), a full breakdown into years, months, weeks, and days, and even the hours and minutes. The Add / Subtract mode enriches the result date with its ISO week number, day of year, quarter, and leap-year status. The Age Calculator tells you not just how old someone is, but how many days they have lived, and exactly when and on which weekday their next birthday falls.
All calculations run entirely in your browser — nothing is sent to a server, and no data is stored.
How it works
Date Difference
Enter a start date and an end date. The calculator subtracts the earlier timestamp from the later one and converts the millisecond difference into days. Toggle "Include end date" to add one extra day to the count — useful when you want to count every calendar day a range spans (e.g. a rental from Jan 1 to Jan 7 is 7 days inclusive, not 6).
Business days are counted by iterating through each date in the range and skipping Saturdays and Sundays. The compound breakdown (years → months → weeks → remaining days) is computed by stepping forward month by month and year by year from the earlier date, borrowing from larger units when the day count goes negative — the same algorithm date libraries use internally.
Add / Subtract
Pick a start date, choose Add or Subtract, enter an amount, and select a unit (days, weeks, months, or years).
- Days / weeks: the result is a simple fixed-duration shift (1 week = 7 days, always).
- Months / years: the shift follows calendar logic. Adding 1 month to January 31 lands on the last day of February, because Feb 31 does not exist. This matches how most people intuitively expect date arithmetic to work.
The result date is enriched with its ISO week number (week 1 = the week containing the first Thursday of the year), day of year (1–366), calendar quarter (Q1–Q4), and whether the result year is a leap year.
Age Calculator
Enter a date of birth and an optional "age as of" date (defaults to today). The calculator steps from the birth date toward the reference date:
- Subtracts full years, then full months, then counts the remaining days — the same breakdown used on official documents.
- Multiplies the total elapsed milliseconds by the inverse of one day to get total days lived, then divides by 7 for total weeks.
- Determines the next birthday by setting the month and day of the birth date to the current or next year, whichever is in the future.
If you were born on February 29 (a leap day), the next birthday is calculated to February 28 or March 1 in non-leap years depending on the platform locale — the tool flags this so there is no ambiguity.
Examples
Days between two dates
Find how many calendar and business days fall between a project start and deadline.
Add 90 days to a date
Calculate what date falls exactly 90 days after today — useful for trial periods, warranties, or legal deadlines.
Subtract 6 months from a date
Find the date exactly 6 months before a contract renewal to set a review reminder.
Calculate age from birthday
Get a precise age breakdown and find out how many days someone has been alive.
Frequently asked questions
How do I calculate the number of days between two dates?
How do I calculate the number of days between two dates?
Enter your start date in the first field and your end date in the second field under the Date Difference tab. The total number of days appears instantly. By default the end date itself is not counted; toggle "Include end date" if you want to count every day the range spans (for example, a hotel stay from check-in to check-out).
What is the difference between calendar days and business days?
What is the difference between calendar days and business days?
Calendar days count every day of the week including weekends. Business days (also called working days) count only Monday through Friday. For example, a range of 10 calendar days that includes two full weekends contains only 6 business days. Use the business days figure for deadlines, payment windows, and contract terms that exclude weekends.
How do I add or subtract days from a date?
How do I add or subtract days from a date?
Switch to the Add / Subtract tab, pick your start date, choose Add or Subtract, enter the number of days (or weeks, months, or years), and select the unit. The result date appears immediately along with its day of the week, ISO week number, and other useful details.
What happens when I add months to a date that falls on the 31st?
What happens when I add months to a date that falls on the 31st?
When the destination month has fewer days than the source day, the date is clamped to the last valid day of that month. For example, adding 1 month to January 31 lands on February 28 (or February 29 in a leap year). This is standard calendar arithmetic and matches the behavior of Excel, Google Sheets, and most programming languages.
How is age calculated precisely?
How is age calculated precisely?
Age is calculated by stepping forward from the birth date to the reference date in whole years, then whole months, then remaining days — the same method used on passports and official documents. The result is expressed as, for example, 34 years, 7 months, 12 days. The total days lived is derived separately from the raw millisecond difference for accuracy.
How do I find out how many days until my next birthday?
How do I find out how many days until my next birthday?
Open the Age Calculator tab, enter your date of birth, and leave the "Age as of" date set to today. The Next birthday panel shows the full date, the day of the week it falls on, and how many days away it is.
What is an ISO week number?
What is an ISO week number?
The ISO 8601 standard defines week 1 as the week containing the first Thursday of the year. Weeks run Monday to Sunday. This means January 1 can fall in week 52 or 53 of the previous year, and December 31 can fall in week 1 of the next year. ISO week numbers are widely used in business scheduling, logistics, and European calendars.
Does the calculator account for leap years?
Does the calculator account for leap years?
Yes. February 29 is recognised as a valid date during leap years. The Add / Subtract result panel shows a Leap year indicator for the result date, so you always know whether the year has 365 or 366 days. The Age Calculator also flags if you were born on a leap day (February 29).
What does "day of year" mean?
What does "day of year" mean?
Day of year is a number from 1 to 365 (or 366 in a leap year) that counts how far into the year a date falls. January 1 is day 1, December 31 is day 365 (or 366). It is used in astronomy, agriculture, logistics, and any field that needs a simple sequential day count within a year.
Is my data sent anywhere or stored?
Is my data sent anywhere or stored?
No. All calculations run entirely in your browser using JavaScript. No dates, ages, or results are transmitted to any server, logged, or stored in any way.
Why does my date show differently in the result?
Why does my date show differently in the result?
The result date is formatted using your browser's locale settings (en-US by default), so it displays as "Wednesday, January 15, 2025". The underlying ISO date (2025-01-15) is always shown below in monospace for unambiguous copy-pasting.
Can I calculate the number of weeks between two dates?
Can I calculate the number of weeks between two dates?
Yes. The Date Difference tab shows total weeks (rounded down to the nearest whole week) alongside the day count. The detailed breakdown panel also shows the remaining weeks and days after subtracting full years and months.
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