rfc2822 April 2001

3.6.1. The origination date field

   The origination date field consists of the field name "Date" followed
   by a date-time specification.

orig-date       =       "Date:" date-time CRLF

   The origination date specifies the date and time at which the creator
   of the message indicated that the message was complete and ready to
   enter the mail delivery system.  For instance, this might be the time
   that a user pushes the "send" or "submit" button in an application
   program.  In any case, it is specifically not intended to convey the
   time that the message is actually transported, but rather the time at
   which the human or other creator of the message has put the message
   into its final form, ready for transport.  (For example, a portable
   computer user who is not connected to a network might queue a message
   for delivery.  The origination date is intended to contain the date
   and time that the user queued the message, not the time when the user
   connected to the network to send the message.)