Edit a date value in plist from terminal
I'm surprised the original solution ever worked, but maybe the defaults command changed in recent versions. Try

defaults write com.test.name.plist TestAppFirstSeen "$(date +'%Y-%m-%d %l:%M:%S +0000')"


Also, %l gives you the hours between 1 and 12. If you want 1 to 24 use %H instead.