Try something like this in your init.sqs file:-
skipTime (random 12)
which should give you a random time between the time you set in the mission editor and 12 hours later. Maybe if you use big enough numbers it will change the date, dunno. If so you just set the date to 1 January in the mission editor. Syntax not guaranteed.
To make a fire come on at a certain time you could use a trigger:
Condition: dayTime > 17.2
On Activation: fire1 inflame true
Not guaranteed, you might need to play with it a little to make it work.
I strongly recommed you read the command reference from start to finish, a bit like a book. Don't go into the commands in detail, just get an idea of what's there. The online version has many helpful Comments and although the unofficial version is a little dated, it sometimes gives a very useful perspective on a command.