Date Calculator

This tool performs calendar-date arithmetic using the Gregorian calendar. It has two modes:

Mode A — Difference between dates

Enter a start date and an end date. The tool returns:

• Total elapsed days — the number of 24-hour periods between the two dates.
• Weeks + days — the same duration expressed as whole weeks and remaining days.
• Years / months / days — a calendar breakdown (see below).
• Direction — whether the end date is before or after the start date.

Mode B — Add or subtract a duration

Enter a start date, choose Add or Subtract, then specify any combination of years, months, weeks, and days. The tool returns the resulting date.

How month arithmetic works

Adding or subtracting months can produce dates that don’t exist (e.g., January 31 + 1 month). This tool uses day clamping: the day is clamped to the last valid day of the target month.

• Jan 31 + 1 month → Feb 28 (or Feb 29 in a leap year)
• Mar 31 − 1 month → Feb 28 (or Feb 29 in a leap year)
• Aug 31 + 1 month → Sep 30

This rule is deterministic and matches the behaviour of most calendar applications. Weeks and days are applied after month/year arithmetic, using exact 7-day and 1-day steps.

Leap years

The Gregorian leap-year rule applies: years divisible by 4 are leap, except centuries not divisible by 400. February 29 exists only in leap years. If a calculation lands on February 29 of a non-leap year, it is clamped to February 28.

Date difference semantics

• Elapsed days — Oct 1 → Oct 2 = 1 day. The count is the number of midnights crossed.
• Same date — difference is 0 days.
• Reversed order — the absolute difference is returned, and the direction is reported.

This tool does not count inclusively (Oct 1 to Oct 2 inclusive = 2 days). If you need inclusive counting, add 1 to the result.

Calendar breakdown (years / months / days)

The years/months/days breakdown uses calendar arithmetic, not division by 365/30. It computes the difference by comparing year, month, and day components directly, borrowing across month/year boundaries as needed. This matches how humans count age or tenure.

Limitations

• This is a calendar-day calculator. It does not support business days, holidays, or work-week masking.
• Time-of-day and timezones are ignored — all inputs are treated as whole calendar dates (YYYY-MM-DD).
• The Gregorian calendar is used for all dates, including those before its adoption in 1582.
• Supported range is roughly 275,000 years on either side of 1970 (JavaScript Date limits).

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