Backward is not forward run in reverse
Counting forward fixes the start and lets the finish float. Counting backward fixes the finish and lets the start float, and for anything with a due date the second is the number you actually need. The construction here begins at the last workable day and adds stages from the end. The final stage ends there; step back its working days to find where it starts; the working day before that becomes the end of the stage before it, and so on to the front. Nothing has to be reconciled at the end because no gaps are ever created, and weekends and days off fall out of the walk rather than needing to be subtracted afterwards.
What counts as a working day
Sunday is always off. Saturday is off unless you switch it on. On top of that sit the days you list yourself and, if you leave the box checked, the eleven US federal holidays, computed from their rules rather than looked up in a table so they stay right in any year. That computation includes the weekend observance federal offices use: a holiday on a Saturday is observed the Friday before, one on a Sunday the Monday after. The federal set is not your set. States add their own, most employers close on days the federal government does not — the Friday after Thanksgiving and Christmas Eve being the usual examples — and plenty of employers work through Columbus Day and Veterans Day. Add the difference to the list yourself. The number you write beside a stage is working days, not calendar days, so a stage of 6 can easily occupy nine days on the wall; the table prints both so the gap is visible.
Non-working days move in two directions
When the end of something lands on a day off, it moves back to the previous working day, because you cannot finish on a day nobody is working. When a start lands on a day off, it moves forward, for the same reason in the other direction: work begins the next day people are there. That is why entering a Saturday as your earliest start reports Monday, and why marking that Monday as a holiday pushes it to Tuesday. Getting this backwards makes a schedule look a day or two roomier than it is, which is the worst direction for the error to run.
Buffer, the due date, and what gives when it does not fit
Whether the due date is itself workable depends on the job. Something filed online by midnight leaves the whole day available; something that has to be handed over at a morning meeting or put in the mail does not, which is why the default is the day before. The buffer is working days deliberately left empty between the last stage and the deadline, and it is where printing, a failed upload, and a reviewer who answers a day late all live. Adding buffer pulls the start date earlier, so it reads as a demand to begin sooner. When the shortfall section says you are short, only four things can move: stage lengths, the buffer, the deadline, or Saturdays. Switching Saturdays on adds one day a week, so four weeks of schedule recovers four days. Be aware that the whole model assumes one stage at a time — genuinely parallel work should be entered as a single line carrying the length of the longer branch.
Questions people ask
Is the number beside a stage calendar days or working days?
Working days. Weekends and anything on the non-working list are not counted in it. A stage of 3 starting on a Friday runs Friday, Monday, Tuesday, which is five days on the calendar. The calendar-days column in the table shows how many nights actually pass.
Are holidays handled automatically?
The eleven US federal holidays are, when the box is checked, computed from their rules with the weekend observance applied. Nothing else is. State holidays and company closures vary too much to guess at, so they go in the list by hand. Uncheck the box entirely if your workplace ignores the federal calendar.
What about two stages that run at the same time?
The model runs stages one after another. Put simultaneous work on a single line with the length of the longer branch: design at 5 days alongside copy at 3 days becomes one line of 5. If the same person does both, though, they are not parallel and should stay as two lines totaling 8.
The start date has already passed. Now what?
That is the shortfall, and the figure it gives you is the useful part of the answer. "We are tight" is a feeling; "we are four working days short, so either the deadline moves six days or review loses a day" is a proposal. Toggle the buffer and the Saturday switch to see how much each one buys back before you take it to anyone.