Whenever a comment is edited, the "created" timestamp still gets changed in two ways:
a) admin edit
While makes sure it isn't completely overwritten, it still gets rounded down to the full minute.
This is because the "Authored on" field in the Admin section only takes
H:i (hh:mm), and this value gets saved to the record, whether it has been changed or not. This won't be a big thing in most cases, but we never know - it's simply not correct.
Probably the "Authored on" field should take
H:i:s (hh:mm:ss), which seems the cleanest way and is consistent to the node edit form.
b) user edit
The second part of wasn't fixed either:
When a non-admin edits a comment, the "created" date is always set to now.
For non-admins the "Administration" section stays hidden. However, the "Authored on" field is fed with an empty string, which upon saving is automatically replaced by
Instead, the created date should be given, if existing (it obviously doesn't exist for new, unsaved comments).
EDIT (@diqidoq): fixd for D8, but not for D7, please read comments #97/#98 why this issue isn't fixed yet. Any thoughts on this are much appreciated.