Timestamp on user edited comments

I have following settings for comments:
- flat - spreaded
- older first
- user can edit their comments
(common behavior for many forums)

When user edits its own comment, it gets stamped with now, and gets on the bottom of the list. Reason for this is that function _comment_form_submit($form_values) asks for form field 'date' but user without 'administer comments' premission has no 'date' field on his form so it gets timestamped again.

I've edited this function, and got desired behaviour, but I'm not pleased with solution. (If ordinary user edit comment, function reads timestamp from database, date format is harcoded into string). Instead, I think ordinary users should have hidden field with date from database, but I didn't know how to do that.