I completely agree! Sometimes I am most productive at night and get on a roll grading, but I personally don't feel like it is professional to submit comments with a timestamp of 2:45 in the morning.
The best workaround I've found is not the most efficient, and probably only makes sense when providing detailed feedback through the comments. When grading late at night, I will go to the gradebook and turn on "manual" grade posting so that the work I am doing is not visible to students until I decide to release all the grades and comments. The next day, at a respectable hour, I go through and copy each comment, then delete it! Then I paste and submit the comment, and the timestamp has changed to the new submission time. Like I said, this is a bit of a waste of time, but taking 10 minutes to mindlessly resubmit comments is worth it to me to fix the timestamp issue.
When done pasting comments, I will go to the gradebook and release all grades (and comments) for that assignment. I THINK this works.