Working days vs calendar days: the same schedule, two answers
Four sequential activities, eight days of work in total, starting Thursday 10 September 2026. Nothing about the logic changes. Only the calendar does: first a normal Monday-to-Friday week, then a round-the-clock shutdown calendar. Predict both finish dates.
| Activity | Duration (working days) | Predecessors |
|---|---|---|
| A Isolate & drain | 1 | — |
| B Inspection | 2 | A |
| C Repair | 3 | B |
| D Reinstate & test | 2 | C |
Durations are worked time, not elapsed time
A three-day activity means three days of work. How long that takes in real life depends entirely on when the calendar is open. This is the single most common mistake in hand-built schedules and in naive online calculators: counting durations as calendar days.
The 5×8 calendar swallows weekends
On a Monday-to-Friday, 8-hour calendar, work that crosses a Saturday simply pauses and resumes on Monday. Two weekends fall inside this eight-day sequence.
The 24/7 calendar does not
A shutdown or turnaround runs continuously. The same eight days of work run straight through the weekend — which is exactly why plants pay overtime rates to shut down over a weekend rather than during the week.
The answer
On the 5×8 calendar the sequence finishes Monday 21 September 2026. On the 24/7 calendar, with identical durations and identical logic, it finishes Friday 18 September. Three calendar days of difference, produced by nothing but the calendar. Switch the Calendar dropdown in the tool below and watch the dates move — the working-day count stays at eight in both cases.
The network, live
This calculator holds the exercise above. Edit anything — durations, links, the calendar — and the dates, float and critical path recompute instantly. Nothing is saved to your own saved project.
Working calendar effect
CPM schedule — start 2026-09-10 — sitraka.lu/checklists/scheduler
| ID | Activity | Type | Dur (d) | Predecessors | Early start | Early finish | TF (d) | |
|---|---|---|---|---|---|---|---|---|
| Thu 10 Sept 08:00 | Thu 10 Sept 17:00 | 0.0 ● | ||||||
| Fri 11 Sept 08:00 | Mon 14 Sept 17:00 | 0.0 ● | ||||||
| Tue 15 Sept 08:00 | Thu 17 Sept 17:00 | 0.0 ● | ||||||
| Fri 18 Sept 08:00 | Mon 21 Sept 17:00 | 0.0 ● | ||||||
Predecessors: B (finish-to-start), B:SS+2 (start-to-start, 2-day lag), B:FF-1, B:SF+3. Lags are in working days on the successor's calendar. Rows highlighted in red are on the longest path. Press Enter in the last row to add an activity.
Project finish
Mon 21 Sept 17:00
Duration
8 working days
Longest path
4 / 4 critical
Critical path
A → B → C → D
4 activities scheduled in 4 ms, entirely in your browser. Your data is saved locally and never uploaded: the “Copy link” button encodes the whole project into the URL itself.
Try it yourself
Move the project start to a Friday and compare again. On the 5×8 calendar almost the whole first weekend is lost; on the 24/7 calendar nothing changes. Start date choice is a scheduling decision, not an administrative one.