Hi John,
I just found your post after encountering the problem myself.
What I'm seeing on my machine is that some MP4 files (not all) do not display the Media Created date, and attempting to set it now gives an error as you describe.
I tried copying one of the problem files to a Windows 8 machine and found that it still had it's original date information and the date could be changed.
So, I'm letting you know this because it seems your data is probably intact, but not being read correctly. You might want to do a similar check yourself before making any significant changes to your data.
My workaround is going to be to set the date in a Virtual Machine running a version of Windows that does not have this problem - hopefully they fix this soon.