Date Calculator
Calculate the number of days, weeks, months, or years between two dates. Also add or subtract days from any date. Free date difference calculator.
Reviewed for accuracy by Marcus Webb and the Blueprint Dynamics editorial team (last updated July 2026). Our calculators use primary-source formulas and are cross-checked against IRS publications, Fannie Mae guidelines, and Federal Reserve data. Learn more about our methodology.
Select both dates to calculate the difference.
What the date calculator computes
The date calculator determines the exact duration between two dates — in days, weeks, months, and years — or calculates a future or past date by adding or subtracting a specified number of days, weeks, months, or years from a starting date. Both functions are more complex than simple subtraction because calendar months have different lengths, leap years add an extra day, and "add 1 month" has no single numeric answer (January 31 + 1 month is February 28 or 29 depending on the year).
Common uses for date duration calculations
Legal and contract timing: lease terms, notice periods, statute of limitations, and contract milestones are often defined in calendar days or months — knowing the exact date they expire is critical. Pregnancy and medical: gestational age is calculated in weeks from the last menstrual period; due date calculations add 280 days (40 weeks). Project management: how many business days between today and a deadline? Scheduling and deadlines: how many days until an event, anniversary, or filing deadline? Financial: bond duration, CD maturity, loan payoff date, warranty expiration. Age verification: how old is someone on a specific future date?
Business days vs. calendar days
Many legal, financial, and business contexts specify deadlines in business days rather than calendar days. Business days exclude weekends (Saturday and Sunday) and federal holidays. The gap between calendar and business days varies depending on how many weekends and holidays fall in the period. A 10-business-day deadline from a Monday adds 14 calendar days; from a Wednesday (if a holiday falls mid-period) it might add 16 calendar days. When precision matters on business-day deadlines, count carefully — and when in doubt, submit early.
Month-based calculations and edge cases
Adding "one month" to January 31 produces different results depending on the system: some use February 28/29 (end of February), others use March 2/3 (31 days after January 31). For lease terms, subscription billing, and recurring contracts, how "monthly" is defined affects which day of the month charges occur and how end dates are calculated. The most common convention is the same calendar day each month, with end-of-month adjustments when a month doesn't have that day — January 31 + 1 month = February 28 (or 29 in a leap year).