class Mail::Message

def add_date(date_val = '')

It will preserve any date you specify if you do.

specifying a date yourself.
DateTime.now's date if you try and encode it or output it to_s without
into the Header. The DateField object will automatically generate
Creates a new empty Date field and inserts it in the correct order
def add_date(date_val = '')
  header['date'] = date_val
end