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Date Calculator — Days Between, Add/Subtract, Business Days

Calculate days between dates, add or subtract days, count business days (excluding weekends and US holidays), and get date info like week numbers and day of year.

Concept Fundamentals
255
Days
Between
177
Business
Days
17
Week #
ISO
110
Day of Yr
Of year
CalculateUse the calculator below to convert and manage time

Why Time Matters

Why: Date calculations are essential for project planning, legal deadlines, payroll, and scheduling. Business days exclude weekends and holidays.

How: Days between: milliseconds ÷ 86,400,000. Business days: iterate dates, skip Sat/Sun and US federal holidays. ISO week: week containing Jan 4.

  • ●252 business days/year
  • ●11 US federal holidays
  • ●ISO 8601 week numbering
Sources:ISO 8601US OPM
📅
ALL-IN-ONEDays, Business Days, Date Math

Date Calculator — Days Between, Add/Subtract, Business Days

Calculate days between dates, add or subtract calendar/business days, and get comprehensive date information including week numbers and holidays.

📅 Quick Scenarios — Click to Load

Calculator Mode

Dates

date_calc.sh
CALCULATED
$ date_diff --from=2026-04-20 --to=2026-12-31
Total Days
255
Business Days
177
Weeks
36
Weekend Days
71
Share:

Days by Month

Day Type Distribution

Calculation Breakdown

DATE RANGE
Start DateMonday, April 20, 2026
End DateThursday, December 31, 2026
RESULTS
TOTAL DAYS255 days
Duration8 months, 11 days
Weeks36 weeks, 3 days
WORK DAYS
BUSINESS DAYS177 (excl. holidays)
Weekend Days71
US Federal Holidays8

For educational and informational purposes only. Verify with a qualified professional.

🕐 Time Facts

252

Approximate business days per year

— US OPM

11

US Federal holidays per year

— OPM

365

Days in a regular year, 366 in leap year

— ISO 8601

Key Takeaways

  • There are 365 days in a regular year and 366 in a leap year (divisible by 4, except centuries unless divisible by 400)
  • A business day excludes Saturdays, Sundays, and typically US federal holidays (11 per year)
  • ISO 8601 defines Week 1 as the week containing the first Thursday of January
  • There are approximately 252 business days per year (365 - 104 weekends - 9 weekday holidays)

Did You Know?

📅The Gregorian calendar (our current system) was introduced by Pope Gregory XIII in 1582 to fix a 10-day drift in the Julian calendar. Britain didn't adopt it until 1752 — people rioted demanding their "11 stolen days" backSource: Royal Museums Greenwich
🔢Leap years occur every 4 years, EXCEPT century years (1900, 2100), UNLESS divisible by 400 (2000 was a leap year, 2100 won't be). The year 2000 was actually a rare 400-year leap yearSource: ISO 8601
💼There are 252 business days in most years. Financial markets use 252 as the standard for annualizing daily returns — each trading day represents about 0.4% of the yearSource: NYSE
🌍ISO 8601 standardizes dates as YYYY-MM-DD worldwide. The US MM/DD/YYYY format is used by only 3 countries. Most of the world uses DD/MM/YYYY or the ISO standardSource: ISO
📊The 13th falls on a Friday at least once every year, and can occur up to 3 times. Statistically, the 13th is slightly more likely to be a Friday than any other daySource: Mathematics
⏰A "business day" varies by country: in Israel, the weekend is Friday-Saturday. In many Middle Eastern countries, it's Friday-Saturday or Thursday-Friday. Always clarify when working internationallySource: World Bank

Frequently Asked Questions

How do I calculate the number of days between two dates?

Subtract the earlier date from the later date. In programming, convert both dates to milliseconds since epoch, subtract, and divide by 86,400,000 (ms in a day). Our calculator handles all edge cases including leap years, month-length differences, and timezone issues.

What counts as a business day?

A business day is any day that is not a Saturday, Sunday, or recognized holiday. In the US, there are 11 federal holidays. Some industries also exclude the day after Thanksgiving and Christmas Eve. Our calculator uses the official US Federal Holiday list from the Office of Personnel Management.

How many business days are in a year?

In a typical year, there are 260 weekdays (52 weeks × 5 days). After subtracting ~8 federal holidays that fall on weekdays, there are approximately 252 business days. The exact number varies year to year based on which days holidays fall on.

What is an ISO week number?

ISO 8601 defines week numbering where Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4th). Weeks run Monday through Sunday. A year has 52 or 53 ISO weeks. December 31 can sometimes be in Week 1 of the following year.

When is the next leap year?

Leap years are divisible by 4: 2024, 2028, 2032, etc. The exception: century years (2100, 2200, 2300) are NOT leap years unless also divisible by 400. So 2000 was a leap year but 2100 will not be.

How do I add business days to a date?

Select "Add/Subtract Days" mode, enter your start date and number of days, and enable "Exclude Holidays." The calculator will skip weekends and US federal holidays, giving you the actual business date. For example, adding 5 business days from a Friday gives you the following Friday (skipping the weekend).

Date Facts by the Numbers

252
Business Days/Year
11
US Federal Holidays
97
Leap Years per 400 yrs
52-53
ISO Weeks/Year

Disclaimer: This calculator uses the Gregorian calendar and US Federal Holiday list. Business day calculations may differ by country, industry, or organization. Always verify critical date calculations (legal deadlines, financial settlements) with official sources.

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