Date Difference Calculator

This date difference calculator (also used as a days between dates calculator, date duration calculator, or date to date calculator) tells you exactly how many calendar days, weeks, months, and years separate any two dates. It is a practical date and time calculator for planners, contracts, travel, and anniversaries, and it handles leap years, month length changes, and weekend math without you having to count squares on a calendar.

1. Enter the start date. Type the date directly (YYYY-MM-DD) or use the date picker. Any year from 1900 through 2099 works, so you can count days from a historical event or forward to a far-future deadline.
2. Enter the end date. This date calculator works in both directions. If the end date is earlier than the start date, the calculator still returns the correct absolute difference, which is useful for backward counting from a deadline.
3. Decide whether to include the end date. The default result excludes the final day, so Jan 1 to Jan 31 returns 30 days. For contract language that counts inclusively (both endpoints), add 1 day to the total.
4. Toggle business days vs calendar days. The result panel shows both. Use calendar days for age, anniversaries, and countdown timers. Use business days for legal deadlines, shipping estimates, and HR policies that say "within 10 business days."

Read the output four ways: total days for raw duration, total weeks plus remaining days for sprint and payroll planning, calendar years plus months plus days for ages and anniversaries, and business days for workday deadlines. All four refer to the same date range, just expressed in different units. Use the quick-preset buttons (1 week, 1 month, 3 months, 1 year) to prefill common ranges from today.

A date range calculator converts both dates to a single absolute number, subtracts them, then translates the result back into the units you want. Here is the exact math this date duration calculator uses.

1. Total days between two dates

Days = (End − Start) in milliseconds ÷ 86,400,000 ms per day
       rounded to the nearest integer

Example: 2025-01-15 to 2025-08-30
  Start  = 1,736,899,200,000 ms (2025-01-15 00:00 UTC)
  End    = 1,756,512,000,000 ms (2025-08-30 00:00 UTC)
  Diff   = 19,612,800,000 ms
  Days   = 19,612,800,000 ÷ 86,400,000 = 227 days

One calendar day is exactly 86,400 seconds, which is 86,400,000 milliseconds. The calculator rounds to an integer to absorb any DST shift (more on that below).

2. Expressing the same range in other units

Weeks            = Days ÷ 7
Years + Months   = calendar arithmetic (not Days ÷ 365)
                   step 1: years  = endYear − startYear
                   step 2: months = endMonth − startMonth
                   step 3: days   = endDay − startDay
                   borrow from months/years if step 3 goes negative

Months and years are not fixed-length, so you cannot just divide total days by 30 or 365. February is 28 or 29 days, other months are 30 or 31, and years are 365 or 366. The calendar-aware method above is what a month calculator or duration calculator uses to produce answers like "7 months and 15 days."

3. Business days (workday subset)

Business Days = count of days in range where
                day-of-week is Mon, Tue, Wed, Thu, or Fri
                (optionally exclude US federal holidays)

Starting from the earlier date, walk forward one day at a time and increment a counter whenever the weekday is Monday through Friday. Saturdays and Sundays are skipped. For strict legal or payroll counting, also skip the 10 US federal holidays: New Year's Day, Martin Luther King Jr. Day, Presidents' Day, Memorial Day, Juneteenth, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving, and Christmas. A pure weekday subset is close to 71.4% of calendar days (5 of every 7).

4. Leap-year rule

A year is a leap year if:
  year mod 4 == 0
  AND (year mod 100 != 0  OR  year mod 400 == 0)

2000 is a leap year (divisible by 400).
1900 is NOT (century year, not divisible by 400).
2024, 2028, 2032 are leap years.
2100 will NOT be a leap year.

This matters any time your range crosses Feb 29. A year from Feb 28 2024 to Feb 28 2025 is 366 days, while Feb 28 2025 to Feb 28 2026 is only 365 days.

Worked example

Start 2025-01-15, end 2025-08-30:

• Total calendar days: 227 days
• Expressed in weeks: 32 weeks and 3 days (227 ÷ 7 = 32 remainder 3)
• Calendar breakdown: 7 months and 15 days (Jan 15 to Aug 15 is 7 months, then 15 more days to Aug 30)
• Business days in range: 163 workdays (227 days minus 64 weekend days)
• Total hours: 227 × 24 = 5,448 hours

Quick reference: common date ranges in days

The two-value day counts reflect how many leap years fall inside the range. A 5-year span that contains one leap year is 1,826 days; one with two leap years is 1,827.

A days between dates calculator sounds simple, but the real-world details (whether to include the last day, whether to skip weekends, what happens across time zones) decide whether the result you return is the one that actually matters. Here is what people most often use this date calculator for, and where the counting quietly goes wrong.

Common uses for a date difference calculator

• Age calculations: exact years, months, and days from birth date to today, or on a specific future date like a graduation.
• Visa and passport validity: confirm you have at least 6 months of passport validity remaining, or count the 90 days of a Schengen visa window.
• Contract and warranty durations: when does a 1-year warranty expire if the purchase was Mar 3, 2025? (Mar 2, 2026, or Mar 3, 2026 depending on inclusion.)
• Insurance waiting periods: many health and disability policies have 30, 60, or 90 day elimination windows before coverage applies.
• Probation and trial periods: 30-day return policies, 90-day employment probation, 180-day lease-break windows.
• Anniversary and milestone tracking: 1,000 days together, 10,000 days alive, or counting down to a wedding or move-in date.
• Pregnancy weeks: a standard 40-week pregnancy from the last menstrual period is 280 days, and most clinical milestones are stated in completed weeks and days.
• Loan and mortgage terms: a 30-year mortgage has 360 monthly payments, but interest accrues every calendar day, so servicer payoff quotes are always dated.

Inclusive vs exclusive end date: did you include the last day?

This is the most common source of off-by-one disputes. A 30-day return policy on an item bought Jan 1 can end on Jan 30 (exclusive counting, the default here) or Jan 31 (inclusive counting). Contracts, tax deadlines, and consumer protection law each pick different conventions, so the difference matters.

Rule of thumb: if the document says "within X days of," that is usually exclusive. If it says "X days from and including," that is inclusive. When in doubt, ask before the deadline rather than at it.

Calendar-day vs business-day durations

A 14-day project is 2 calendar weeks and spans one weekend. A 14-business-day project is 2 full work weeks plus 4 more workdays, which is roughly 20 calendar days or about 3 calendar weeks. Mixing these up is one of the most expensive mistakes in project scheduling.

Federal holidays further shorten the business-day count by up to 10 per year. For a SEC filing window or an IRS response deadline, always verify whether the rule counts "days" (calendar) or "business days" (weekdays excluding federal holidays).

Time zone and DST gotchas

Two dates picked in different time zones can produce the same or different day counts depending on how the boundary is drawn. A flight that leaves New York at 23:00 on Mar 1 and lands in London at 10:00 on Mar 2 looks like a 2-day span on a New York calendar and an 11-hour span on a wall clock, but the date-difference math treats it as 1 calendar day. This calculator normalizes both inputs to midnight in the same zone, so the day count is stable.

Daylight saving time does not add or subtract calendar days. A day in March when clocks spring forward is 23 hours long, and a day in November when clocks fall back is 25 hours long, but both still count as one calendar day in a date duration calculator. The millisecond math above is rounded to an integer precisely so a DST shift cannot create a phantom half-day.