How date difference calculator Works
Calendar days vs working days
A 200-day span is 143 working days — the difference between a project that "has six months" and one that has 28 Fridays. Every deadline, lead time, and "weeks until" conversation runs on one of the two calendars; the calculator reports both so the ambiguity dies on the page.
The since/until framing
The same arithmetic answers two questions: how long SINCE (anniversaries, sobriety counts, tenure) and how long UNTIL (deadlines, retirements, launches). The calculator detects direction and frames the headline accordingly — the math is symmetric, the meaning is not.
Month counting conventions
Months between dates follow the borrow convention (like age): full calendar months completed, remainder in days. Different tools count differently (30-day months, exact calendar months) — variance of a day or two at the edges. For contracts and legal counting, the governing document's definition wins; for planning, calendar-exact is the standard.