Leap years skip a beat
Years divisible by 4 are leap years—except centuries, which need to be divisible by 400. That is why 2000 was a leap year but 1900 was not.
Leap logic
Date Calculator — Days Between Dates & Add/Subtract (Business Days)
1) Days Between Dates
Tip: Business days ignore public holidays.
2) Add / Subtract Time
Month/year arithmetic is end-of-month clamped; leap years handled automatically.
About This Date Calculator
Dates are easy to read but surprisingly tricky to calculate. This Date Calculator helps you answer two common questions in seconds: how many days are between two dates, and what date you get after adding or subtracting a period of time. It is built for everyday planning, from personal schedules to professional timelines, and it runs entirely in your browser for speed and privacy.
The first mode counts the days between two dates. You can use inclusive counting when both the start and end date should be counted, such as a two-day event, or exclusive counting when only the days in between matter. If you only want weekdays, turn on the business days option to exclude weekends. This is useful for tracking workdays or estimating turnaround time.
The second mode adds or subtracts time from a base date. You can move forward or backward by days, weeks, months, or years, with an optional “skip weekends” toggle for weekday-only schedules. When you add months or years, the calculator automatically clamps to the last valid day of the month to avoid invalid dates. For example, adding one month to January 31 lands on the last day of February, and leap years are handled for you.
How to use it step by step: choose the calculation type, enter your start and end dates (or a start date and a
time offset), select any weekday-only options, then click Calculate to get the result. You can use Set Today to
fill in today’s date quickly. The results appear in a clean ISO format (YYYY-MM-DD) so they can be
copied into spreadsheets, reports, or calendars without confusion.
Real-world uses include planning project deadlines, measuring probation or notice periods, calculating rental spans, scheduling study plans, or estimating delivery windows. If you are asked to “add 45 days” or “find the number of days between two dates,” this calculator provides a clear answer without manual counting.
Under the hood, date math is performed at midnight UTC to reduce daylight-saving surprises across regions. If you need holiday-aware calculations, you can subtract those dates manually or use a dedicated business days tool with holiday lists. This calculator keeps the process fast, simple, and dependable for everyday date arithmetic.
Tip: use the Set Today shortcut to prefill fields with today’s UTC date and accelerate common workflows.
Date Calculator: FAQs
What does Inclusive mean in day counting?
Inclusive counting includes both the start date and the end date in the total. Exclusive excludes one endpoint.
How are business days calculated?
Business days count Monday through Friday only and ignore public holidays. Weekend days are excluded.
Does the calculator handle months and leap years?
Yes. Month and year arithmetic is clamped to end-of-month, and leap years are respected when rolling dates.
Is my data private?
Yes. All calculations run locally in your browser. No data is uploaded or stored on a server.
5 Fun Facts about Dates & Calendars
Months got their names from people
July and August are named after Julius Caesar and Augustus. Two “long” months in a row? That is a calendar flex from ancient Rome.
Calendar history
Weeks start on Monday (for ISO)
In ISO 8601, week 1 is the week with the first Thursday of the year. That is why early January days can still belong to the last week of the previous year.
Week math
Some days are 23 or 25 hours
Daylight saving shifts can shorten or lengthen a day. Using UTC avoids the “missing hour” problem when you are just counting dates.
Timezone quirks
End-of-month rules are surprisingly common
If you add one month to January 31, you land on February’s last day. Many billing and contract systems use this same clamp.
Date math