Editing previous posts sounds like a massive headache and a lot of work prone to errors and mistakes. But a time travel rp has potential. 

I'd suggest simply making it explicit and clear the time and date of a post within the IC, and then frequently updating the op of an rp with a timeline of dates (and if neccessary, times during those days) along with a summary of what happened there, as an easier guide and reference. For any changes that occur, make it clear in the summary how that branches off and what new events happen (ala parallel worlds).