Nice!

Note that upgrading timestamps is optional. The calendars will make the data available indefinitely, and third parties can back it up, so it's reasonable to just store the incomplete ones in your databases.

The way I'd implement it is to timestamp the content after the edit window has closed. Also, ideal would be for the SHA256 hash of what you're replying too also be part of the timestamped content, so there's a complete record of the context. Don't forget to hash content like inline photos too.