Enter values to see results
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days
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weeks
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weeks
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months
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years
Enter values to see results
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days
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weeks
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weeks
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months
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years
The Date Difference Calculator computes the elapsed time between any two dates and expresses it simultaneously in days, weeks, months, and years. This all-in-one tool eliminates the need for separate conversions and provides immediate multi-scale context for any date interval.
Date differences are fundamental to countless domains: project management uses day counts for sprint planning and deadline tracking; legal contracts define payment terms, notice periods, and statutes of limitations in days; financial instruments use day counts for accrued interest calculations (actual/360, actual/365, 30/360 day count conventions); medical treatment protocols define intervals in days between doses or follow-up visits; and historical research computes durations of events, reigns, and periods.
The calculator uses the absolute difference — it returns a positive result regardless of which date is entered first. The total days figure uses exact calendar arithmetic, while months and years use standard Gregorian averages (30.4375 days/month, 365.25 days/year) as widely accepted approximations. For exact year and month decomposition, use the Age Calculator or Years/Months Between calculators.
The calculator computes the absolute millisecond difference and converts to four time units:
$$\Delta t_{ms} = |t_{end} - t_{start}|$$
$$\text{Total Days} = \left\lfloor \frac{\Delta t_{ms}}{86{,}400{,}000} \right\rfloor$$
$$\text{Complete Weeks} = \left\lfloor \frac{\text{Total Days}}{7} \right\rfloor$$
$$\text{Total Months} = \left\lfloor \frac{\Delta t_{ms}}{2{,}629{,}800{,}000} \right\rfloor$$
where 2,629,800,000 ms is the average month (365.25/12 days × 86,400,000 ms/day).
$$\text{Years} = \frac{\text{Total Days}}{365.25}$$
The absolute value ensures symmetric behavior: swapping start and end dates yields identical results. This is appropriate for duration calculations where direction (past/future) is not relevant — only the span length matters. The day count convention used here is Actual/Actual (ISDA), the standard for most financial calculations per ISDA 2006 Definitions.
Total Days is the authoritative, exact figure — the number of calendar days in the interval. Complete Weeks is the whole-week count (ignore partial final week). Total Months uses a 30.4375-day average month, appropriate for estimates. Years is a decimal using the 365.25-day average year. For a 2-year interval: Days ≈ 730-731, Weeks = 104, Months ≈ 24, Years = 2.00. For a 6-month interval: Days ≈ 181-184, Weeks = 25-26, Months = 6, Years ≈ 0.50. If start and end dates are the same, all outputs are zero. The four outputs together give a comprehensive picture of the interval's magnitude across all common time scales.
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Results
September 1, 2020 to June 15, 2024: 1,383 days, 197 complete weeks, approximately 45 months — just under 4 academic years (3.79 years).
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Results
The first UK COVID-19 lockdown (March 23 to July 4, 2020): 103 days, 14 complete weeks, approximately 3 months, 0.28 years.
No. The calculator uses the absolute difference — it returns the same result regardless of which date is entered as start or end. Entering January 1 as start and December 31 as end gives the same result as the reverse. This is appropriate for duration calculations where you care about the length of the interval, not its direction. If you need to determine whether a date is before or after another (for scheduling or deadline purposes), the Days Between Dates calculator provides directionally sensitive output.
Financial instruments use specific day count conventions that differ from simple calendar day counts: Actual/Actual (ISDA) — counts actual calendar days, divides by 365 or 366 (used in government bonds); Actual/360 — counts actual days but divides by 360 (used in US money markets, Eurodollar deposits); Actual/365 Fixed — counts actual days, always divides by 365 (used in UK gilt markets); 30/360 — assumes each month has 30 days and the year has 360 days (used in US corporate bonds). The day count from this calculator is "Actual" — the raw day count. The denominator choice depends on the financial instrument's convention.
Calendar days count every day including weekends and public holidays. Business days (or working days) exclude weekends (Saturday and Sunday in most countries) and may also exclude public holidays. Legal notice periods, contract payment terms, and court filings often specify one or the other. A 30 calendar day notice period starting Monday January 1 ends on January 31 (including weekends). A 30 business day period starting the same day ends on approximately February 13 (adding back ~10 weekends). For business day calculations, use the Business Days Calculator.
The months output uses an average month of 30.4375 days (365.25 / 12). This is a standard actuarial and demographic approximation, but it may differ from exact calendar month counting. For example, the interval from January 31 to March 1 is 29 days (28 in non-leap years), but the average-month formula would yield 0 months for 28 days and 0 months for 29 days (since 29 < 30.4375). For exact calendar month counting, use the Months Between Dates Calculator which uses the birthday-counting algorithm.
Legal systems frequently specify intervals precisely: statutes of limitations (e.g., 3 years from the date of injury to file a claim), notice periods (e.g., 30 days written notice required for lease termination), appeal deadlines (e.g., 28 days from judgment), probationary periods (e.g., 90 days from employment start), and contract validity periods. Courts interpret "days" as calendar days unless "business days" is specified. The exact day count from this calculator is directly applicable to such legal interval computations. Always confirm with legal counsel for specific applications.
Yes. JavaScript's Date object implements the full proleptic Gregorian calendar, including the special leap year rules: a year is a leap year if divisible by 4, EXCEPT centuries (divisible by 100) are NOT leap years, UNLESS also divisible by 400. So 1900 was NOT a leap year (divisible by 100 but not 400), but 2000 WAS (divisible by 400). This affects day counts spanning these years. The calculator correctly handles intervals crossing any century boundary. For intervals stretching back before 1582 (when the Gregorian calendar was adopted), note that the proleptic Gregorian calendar is used retroactively, which may not align with historical Julian calendar records.
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