Date Difference Calculator

What this tool is used for

• Getting the gap between two dates in several units at once.
• Working out a tenure or a service length.
• Checking a contract period's duration against what the terms actually say.
• Counting whole weeks between two milestones for a sprint plan.
• Confirming a duration against a system's own figure.

Frequently Asked Questions

Why is a difference in months ambiguous?

Because months have different lengths. From 31 January to 28 February is one month or 28 days depending on how you ask, and adding one month to 31 January has no exact answer at all. Only differences in days and below are unambiguous.

Why do years, months and days not simply add up?

Because the breakdown depends on the order of operations. '1 year, 2 months and 3 days' is computed by consuming years first, then months, then days — and computing it in a different order gives a different-looking but equally correct answer.

Should the count include both endpoints?

State which you mean. Payroll and rentals usually count inclusively; age and duration exclusively. A one-day discrepancy in a contract term is not trivial, so the convention belongs in writing rather than in the code alone.

What about dates before 1582?

The Gregorian calendar began in October 1582, and adoption ran to 1923 in some countries — Britain skipped 11 days in 1752. Date libraries use a proleptic Gregorian calendar, so historical dates may not match contemporaneous records.

How do I count business days between dates?

Exclude weekends and the relevant public holidays. Note the weekend itself is not universal — Friday-Saturday in much of the Middle East — so a hardcoded Saturday-Sunday assumption is a real internationalisation bug.

How is an age or duration in years and months conventionally computed?

Years first, then months from the remaining part, then days — borrowing from the earlier month when the day of month is smaller. Computing each unit independently double-counts, which is why the three parts must be taken in order.

Why can adding a month and subtracting it not return the original date?

Because month lengths differ and the result is clamped. 31 January plus one month is 28 February, and subtracting a month gives 28 January. Calendar arithmetic is not reversible, which surprises most code that assumes it is.

Common errors and gotchas

• Reading a years-and-months result as unambiguous, when month lengths make it convention-dependent.
• Counting both endpoints when the interval is exclusive.
• Mixing time zones between the two dates.
• Ignoring daylight saving, which changes an hours figure across a transition.
• Assuming a month means 30 days, which it does for no month in particular.