Dates, times, actions, events, details and whatever else your BH might want to know. The timeline is generally that one time when you can get it all out there in writing. It's your "free pass" even though it isn't a free pass.
Some BS's want every single detail, some don't. Whatever you do, just stick to the facts. Don't put in reasons why you did this or that, just put down what happened when and where.
I would suggest that you ask for the option to amend it as some possibly forgotten details might come back to you. But make sure that you don't TT either (if that makes any sense...). There isn't really a format.