Date Difference Calculator

Calculating the difference between two dates is more complex than it appears. A simple millisecond subtraction gives precise duration, but converting that into years, months, and days requires accounting for varying month lengths (28 to 31 days), leap years, and whether the calculation should follow calendar rules or use exact durations. The distinction matters: the difference between January 31 and March 31 is 2 calendar months, but the exact number of days varies depending on whether it is a leap year.

Leap years follow a rule with three conditions: a year is a leap year if it is divisible by 4, except for years divisible by 100, which are not leap years, unless they are also divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be. This rule matters when computing date differences that span February in years near these boundaries.

Time zone awareness becomes important when comparing dates that cross DST transitions. A day during a DST 'spring forward' is only 23 hours long; a day during a 'fall back' is 25 hours. If you compute the difference in hours and convert to days, you may get a different result than counting calendar days. For date arithmetic in applications, always use a well-tested date library rather than manual arithmetic.