The Date field inherits from StructuredField and handles the Date: header field in the email.
Sending date to a mail message will instantiate a Mail::Field object that has a DateField as its field type. This includes all Mail::CommonAddress module instance methods.
There must be excatly one Date field in an RFC2822 email.
mail = Mail.new mail.date = 'Mon, 24 Nov 1997 14:22:01 -0800' mail.date #=> #<DateTime: 211747170121/86400,-1/3,2299161> mail.date.to_s #=> 'Mon, 24 Nov 1997 14:22:01 -0800' mail[:date] #=> '#<Mail::Field:0x180e5e8 @field=#<Mail::DateField:0x180e1c4 mail['date'] #=> '#<Mail::Field:0x180e5e8 @field=#<Mail::DateField:0x180e1c4 mail['Date'] #=> '#<Mail::Field:0x180e5e8 @field=#<Mail::DateField:0x180e1c4