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Reviewed by CalculatorApp.me Editorial Team
Deadlines, countdowns, durations, and leap years — all answered precisely.
365.25
Average days per Gregorian year
97 / 400
Leap years per 400-year cycle
86,400
Seconds in one day
ISO 8601
International date standard
Calculate the exact duration between two dates in years, months, days, hours. Includes business days, weekend count, and historical comparisons.
Enter values above to see results.
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A date calculator computes the span, difference, or future/past date between two points in time. Unlike simple subtraction, real date math must account for varying month lengths (28–31 days), leap years, time zones, and optional exclusion of weekends or holidays.
Common use cases include counting days until a deadline or event, calculating age precisely in years/months/days, determining contract expiry dates, computing gestational duration, tracking subscription periods, and verifying court filing deadlines.
Our calculator uses proleptic Gregorian calendar arithmetic — the standardized system used by ISO 8601, programming languages, and international law.
Days = |Date₂ − Date₁|
Both dates converted to
Julian Day Number first:
JDN = 367Y − ⌊7(Y+⌊(M+9)÷12⌋)÷4⌋
+ ⌊275M÷9⌋ + D + 1721013.5Julian Day Numbers eliminate month/year boundary confusion.
if (year % 400 == 0): LEAP elif (year % 100 == 0): NOT LEAP elif (year % 4 == 0): LEAP else: NOT LEAP Examples: 2000 ✓ 1900 ✗ 2024 ✓
The 400-year rule was introduced by Pope Gregory XIII in 1582.
new_month = (month + m − 1) % 12 + 1 year_carry = (month + m − 1) ÷ 12 new_year = year + year_carry Clamp day to max days in new_month
Month addition must clamp day (e.g., Jan 31 + 1M = Feb 28/29).
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋
+ ⌊J/4⌋ − 2J) mod 7
0=Sat, 1=Sun, 2=Mon … 6=Fri
Jan/Feb treated as month 13/14
of prior year| Calendar | Year Length | Months | Leap Rule | Used By |
|---|---|---|---|---|
| Gregorian | 365.2425 days | 12 | ÷4 except ÷100, unless ÷400 | Most of the world |
| Julian | 365.25 days | 12 | ÷4 always | Orthodox Christian liturgy |
| Hebrew | ~354–385 days | 12–13 | 7 leap yrs / 19-yr cycle | Jewish religious use |
| Islamic (Hijri) | 354–355 days | 12 | No solar correction | Muslim world (religious) |
| Chinese | ~354–384 days | 12–13 | Intercalary month added |
Julius Caesar reformed the Roman calendar, introducing a 365.25-day year with a leap day every 4 years — a vast improvement over the chaotic Roman republican calendar.
Pope Gregory XIII corrected Julian calendar drift (10 days by 1582) with the modern leap-year rule. Catholic countries adopted it immediately; Protestant countries waited decades.
Britain skipped 11 days (Sep 2 → Sep 14, 1752). American colonists were part of this change — Washington celebrated two birthdays to acknowledge the shift.
The International Meridian Conference standardized Greenwich as 0° longitude, creating the foundation for coordinated universal time zones.
ISO
Defines the internationally unambiguous YYYY-MM-DD format and duration notation (P1Y2M3D).
IERS
Defines leap seconds and UTC corrections. The Earth's rotation is irregular; atomic time must periodically sync to solar time.
RFC 5545
Defines the open standard for calendar data exchange used by Google Calendar, Apple Calendar, and Outlook.
USNO
Official source for sunrise/sunset, moon phases, and Julian dates used in navigational and scientific calculations.
You can find days between dates by just subtracting the numbers.
You must account for varying month lengths and leap years. A naive subtraction gives wrong answers at month/year boundaries.
Every 4 years is a leap year.
Years divisible by 100 are NOT leap years unless also divisible by 400. So 1900 was not, but 2000 was a leap year.
February always has 28 days.
February has 29 days in leap years — which occur in approximately 24.25% of all years.
Time zones don't matter for date math.
A meeting scheduled for January 1 at 1:00 AM EST is still December 31 in Los Angeles. UTC normalization is critical for cross-timezone deadlines.
Age, duration, countdowns, timezone converters — every date tool in one place.
Browse All Tools →Last updated:
Zeller's formula computes day-of-week with no lookup tables.
| Traditional East Asia |
| ISO 8601 | N/A (standard) | N/A | Gregorian base | International standards |
Unix systems defined January 1, 1970 00:00:00 UTC as time zero. Most modern programming languages and databases count seconds from this point.
The International Organization for Standardization published ISO 8601, defining the YYYY-MM-DD format to eliminate date ambiguity across cultures and systems.