Calculate the days between dates, count business days, or find any date after adding or subtracting days, weeks, and months. Free date difference calculator — instant results.
Date difference & date arithmetic
This date calculator has two modes: Date Difference, which calculates how many calendar days, business days, weekdays, and weeks fall between any two dates; and Add/Subtract Days, which finds the exact date after adding or subtracting any number of years, months, weeks, or days from a starting date. Both modes work for any past or future dates — from calculating how many days until a vacation to figuring out what date falls 120 business days from contract signing. Leap years are handled automatically, so February 29 is correctly included when applicable.
US workers and project managers use the business days calculator constantly: a contract with "30 business days to close" means approximately six calendar weeks, not 30 calendar days. HR departments track payroll cycle dates, benefits waiting periods, and FMLA leave durations in business days. The IRS uses calendar days for tax deadlines — your return is due 3 months and 15 days after the close of your fiscal year, and the day-by-day count matters for late filing penalties. This calculator makes all of these computations instant, removing the manual counting that leads to mistakes.
Calculate exact day counts between any two dates — birthdays, anniversaries, project deadlines, or retirement dates. Results include total days, weeks, weekdays, and weekend days all at once.
A typical US work year contains about 260 business days. Contract terms, court deadlines, and banking transactions often specify business days — this mode excludes weekends for accurate professional calculations.
Find out what date falls 45 days from today, or what day of the week January 1, 2027 falls on. Useful for calculating due dates, lease expirations, warranty end dates, and subscription renewals.
IRS tax deadlines, FMLA leave tracking, contractor milestone dates, visa expiration countdowns, mortgage rate lock periods — this calculator handles them all by simply entering your start and end dates.
Counting days between dates is deceptively awkward because months have unequal lengths and leap years follow a three-part rule: every year divisible by 4 is a leap year, except centuries, unless the century is divisible by 400. That is why 1900 was not a leap year and 2000 was. The reliable computational approach converts each date to a serial day number — a count of days from a fixed epoch — and subtracts, which sidesteps month lengths entirely. Whether the endpoints are included is a convention that must be stated: hotel nights, loan interest and holiday entitlements each count differently.
Days between = serial(date₂) − serial(date₁)Inclusive count = difference + 1Leap year if (year mod 4 = 0 and year mod 100 ≠ 0) or year mod 400 = 0where:
Assumptions: Calendar days, not business days. Business-day counts require a weekend rule and a holiday calendar, both of which are jurisdiction-specific.
Count the days by month, then convert to weeks and check the leap-year status.
Result257 days — 36 weeks and 5 days
Counted inclusively — treating both the start and end date as elapsed days — the answer is 258. The distinction matters for contractual deadlines, where "within 30 days" and "by the 30th day" can mean different dates.