About

Project managers and logistics coordinators often need to determine specific dates based on lead times or contract terms. This calculator performs robust calendar arithmetic, handling leap years, varying month lengths, and business-day logic automatically.

Miscalculating a legal deadline by one day can result in contract nullification. This tool uses the standard Gregorian calendar system. The "Business Days" mode excludes Saturdays and Sundays from the count, simulating standard Western work weeks.

Formulas

When adding m months to a date D, if the resulting month has fewer days than the original day d, the result is clamped to the last day of that month:

result = min(daysInMonth, d)

For business days, the sequence iterates excluding index 0 (Sun) and 6 (Sat):

{

+1 if day ∈ [Mon..Fri]+0 otherwise

Reference Data

Unit	Definition	Notes
Year	365 or 366 days	Leap years occur every 4 years (mostly).
Month	28, 29, 30, or 31 days	Calculations preserve day-of-month if possible (clamped to month end).
Week	7 Days	Constant.
Business Day	Mon-Fri	Excludes Sat/Sun. Holidays not auto-excluded unless specified.
Solar Day	24 Hours	Based on mean solar time.

Frequently Asked Questions

The tool uses standard calendar logic. Adding 1 month to Jan 31st results in February 28th (or 29th in a leap year), as February does not have a 31st day.

No. The "Business Days" filter strictly removes Saturdays and Sundays. Since holidays vary globally by region and jurisdiction, manual adjustment is recommended for specific national holidays.

The JavaScript Date object supports dates approximately 285,616 years forward or backward from 1970. However, historical accuracy before the Gregorian adoption (1582) may vary.