the subject has a "Subject:" right before it and the sender a "Sender:", now, to be consistent the date should have a "Date:" before it.
This is the design. We decided it looked better with out the Date header. And it frees up more real estate for long subject and from fields. It's pretty obvious that it's the date too so there is no loss of information. In short, we designed it this way =).
Mark it verified/dup.