I have also voted for this Tracker item.
I completely understand the need for this fix. The "Display period, Display start" date is the date that should be shown as the "Started by" date. I cannot see any good reason why anyone would see this differently. This fix will support the ability to prepare posts ahead of time, which your TA's want to be able to do. I too would use this feature, since I like to play ahead. Right now, I purposely have to wait until I turn a weekly topic on for students before making posts, just to avoid this "posted date" problem.
I am not a programmer, so I cannot help with the code to make this work. But the logic should be:
If Display_Start is enabled, then Post_date=Display_Start_Date.
Well, that's my simple-minded approach. I suspect if someone perceived the need to use the actual post date, that another option could be provided saying something like: "Use display_start as post_date" YES/NO, with the default being YES.