Date Difference Calculator

Method: the total day and week counts are computed with simple subtraction — the absolute number of days between the two dates, divided by 86,400,000 milliseconds per day and rounded — and dividing that day count by 7 (to one decimal place) gives the week count. The calendar breakdown uses the same borrowing subtraction as the Age Calculator: the earlier of the two dates is always treated as the "start" and the later as the "end," so years/months/days = end − start with borrowing applied whenever a component would go negative, exactly as when calculating a birthdate-to-today age. Worked example 1: Date A = 5 March 2026, Date B = 20 November 2026. Sorting gives start = 5 March, end = 20 November. Days: 20 − 5 = 15. Months: 11 − 3 = 8. Years: 0. Total days: 260 (37.1 weeks). Result: 0 years, 8 months, 15 days; 260 total days; 37.1 total weeks; "Date A is earlier." Worked example 2 (crossing a year boundary): Date A = 20 November 2026, Date B = 10 January 2027. Start = 20 November 2026, end = 10 January 2027. Days: 10 − 20 = −10, so borrow a month (December has 31 days): −10 + 31 = 21 days. Months: after the day borrow, the month count needs its own borrow across the year boundary, resolving to 1 month. Years: 0 after the year borrow. Result: 0 years, 1 month, 21 days; total days = 51 (7.3 weeks); "Date A is earlier." Edge cases handled: - Dates entered in reverse order (Date B earlier than Date A): the tool detects which is actually earlier before computing the calendar breakdown, so the years/months/days figures are always positive and meaningful. - Identical dates: reported as "Same date" with all difference figures at zero, rather than a blank or an error. - Leap years inside the range: the day count is always exact because it is derived from real calendar dates, so a range spanning 29 February is automatically one day longer than the equivalent range in a non-leap year — no manual adjustment needed. - Very large gaps (decades or centuries apart): the arithmetic works identically for any date range, since JavaScript's Date object handles a very wide range of year values without special-casing. Practical tip: when quoting a duration in a formal context (contracts, invoices, notice periods), prefer the exact day count over the calendar breakdown, since "8 months" can be ambiguous about whether it means 8 calendar months or roughly 243 days — the day count removes that ambiguity entirely. Use the calendar breakdown for human-readable summaries ("almost 9 months") where exactness matters less than intuitive framing.