When does it actually finish?
Forwards from a start, backwards from a deadline, or the days between two dates — on the week you actually work, with holidays out.
Working days, on the week you actually work
Solve forwards or backwards, on a configurable week, with holidays and shutdowns taken out.
from..to for a shutdown period.Which days counted
How this counts
A duration of one working day means the work happens on that day. Starting Monday with a one-day task finishes Monday, not Tuesday. This sounds obvious and is the single most common off-by-one in project dates — internally the finish is the instant work stops, and the date shown is the last day actually worked.
A start on a non-working day moves forward to the next working day rather than being counted as begun.
Backward solving is not simply forward with a minus sign when holidays are involved, so it is computed by stepping backwards through the calendar rather than by subtracting.
Dates are handled in UTC so that a working day does not change identity across a daylight-saving boundary.
The working week is not always five days
Mon-Fri is a default, not a fact. A Sunday-to-Thursday week is standard across much of the Middle East, four-day weeks are increasingly common, and construction and manufacturing frequently work six. A date calculator that assumes Mon-Fri quietly produces the wrong answer for everyone else, and the error compounds with every week of duration.
Why the off-by-one happens
There are two reasonable meanings for "finish": the last day worked, and the moment work stops. They differ by one day, and both are used in practice — scheduling software mostly means the second, people mostly mean the first.
Mixing them is what produces plans where a five-day task starting Monday finishes "Saturday". This tool computes in the unambiguous form and converts once, at the end, to the day you would actually write on a plan.
Holidays make backward planning different
Counting forward and counting backward are not mirror images once holidays are involved. Ten working days after a date and ten working days before another date step through different holidays, so a backward answer derived by subtraction can be off by several days. Backward solving here walks the calendar in reverse rather than negating a forward result.