The persistence of time can be easily summarised as such: Read (the past), Read/Write (the present) and Write (the future).
Since we have no proof to oppose this statement, we can say that the past is "read only", in essence, it can’t be changed.
What is done is done.
However, we can time travel back.. just go to a retirement village.. they live in the past quite comfortably. Seriously, for a second; I believe that all matter, matters. In inanimate objects which are old, we can observe echoes of the past through the effects of exposure to events, over time. A torn page in a book, is an echo of an event where the page was torn, etc. Some signs may be far more subtle.
Time travel is possible.. Heck, you could time travel right now – just go bang your head on something and get knocked out. When you come around, you’ll have jumped ahead a number of hours – time travel is fun, because time is relative.
Why is the present tense read-write? Mainly because you can choose to pursue the past (look through a photo album or watch old home movies, i.e. live in the [past] read-only) or you can create new memories (go sky diving, live in the moment i.e. write the future).
Anyhow, just some random thoughts..