In the programming world, giving deadlines is much harder than in every domain you posted up there.
Why? Because the complexity of the problems that may show up are way over the current state of the human brain's capacity of prediction.
In Agile development, for example, there is Kanban, a method of working. This is based on doing as much as you can, but not exactly predict when it is done.
Also, it also adds if they work on the project part time.

Again, prediction can be done, but not precisely, and people might have other matters on their heads...

Nobody gives you any warranty, don't buy if you don't trust, just stop flaming.