How dates with legal consequences are worked out
Cooling-off, finance, building and pest, settlement. The product works these out for you.
How it works
There is exactly one place in the product that counts days. Every screen that shows a legally meaningful date asks that one place, and nothing else does the arithmetic.
That sounds like an implementation detail and it is the whole point. Day counting has rules that differ by state and by clause: some counts include Saturday, some do not, some skip public holidays, some skip only the ones in a particular place. Two pieces of code written months apart will implement two of those rules and both will look right.
What proves it worked
Open the same deadline from two places, the deal and the diary. Same date. Then open the date itself: it shows the rule that was applied, the jurisdiction, and the calendar of non-counting days it used.
If two screens ever show two dates for the same deadline, that is a serious fault and worth a ticket immediately.
The calendars
Public holidays differ by state, and some differ by region within a state. The calendars are data, kept per jurisdiction, and a date is computed against the one for the property rather than the one for your office.
Changing a date
You can override a computed date where the contract says something the rules do not cover, which happens. An overridden date is recorded as overridden, with who did it and why, and it stops being recomputed.
That record is the useful part. A date that quietly stopped matching its rule is the one that is hard to explain later.
What this is not
It is not legal advice, and a computed date is not an opinion about your contract. The product applies the counting rule to the dates you gave it. If a clause is unusual, the answer comes from the contract and your adviser, and the override exists for exactly that.