There is no way to do this that I'm aware of in Premiere Elements.
However, check out a program called DV Date. It should be able to help you. Though it works primarily with tape-based video, it may also work with AVCHD.
PrE lacks the TimeCode Effect (unless it has been added in PrE 10?), that both PrPro and Adobe After Effects has. However, even that Effect does not do date/time, just Runtime TimeCode, but with some display alternatives.
There are two programs called DVDate. One is/was freeware, and the other was inexpensive shareware. I do not have the two links handy, but believe that either will do what the user wishes.
I know that in the past, one valued contributor, Neale, has posted the links, and has also given the details of what each of the programs can, and cannot do. Maybe he'll drop by this thread, and fill in the blanks that I am leaving here.
We could update to PrE 10 if that would work.
I didn't want to have to have someone learn a different program if at all possible.
Thanks for the posts though and maybe someone can answer if PrE 10 allows us to turn on the TimeCode effect for already created videos and new ones.