I totally agree with the frustration. I often have need to copy and paste the basic header info – and 'basic' includes the date.
Before Mavericks version of Mail (curently v7.2) I could copy and paste in a flash.
The current workaround is very cubersome: manually type every character of the date.
Even turning on full headers does not allow the date to be copied. This must be fixed. Doesn't anyone have a fix? perhaps some esoteric code in Termnal to turn on the copying of the date along with everything else?
How was this major oversight missed?