Date and Time of a post should not change if you edit it. The original date+time should be preserved, and it should automatically add the date and time of the (most recent) modification.
It could look something like this: Feb 12, 2015 12:23 PM [Modified March 2022 2:45 PM]
This would make it easier to decipher some threads. I tend to fine tune, and there are times when I don't add notes to my old posts simply because it would throw it out of chronological order, and make the series of replies confusing. Also, on principle, changing the date stamp is wrong.
There is currently an optional message to indicate changes were made. It claims to say When changes were made, but that would be redundant since the original date+time is overwritten (so the original date is the missing info).
[minor edit March 10, 2015 to test effect on date]