Format timestamp according to RFC 5322 as described in RFC 5321 (chapter 4.4):
Format timestamp according to RFC 5322 as described in RFC 5321 (chapter 4.4):

Time-stamp-line = "Received:" FWS Stamp <CRLF>
Stamp = From-domain By-domain Opt-info [CFWS] ";"
FWS date-time
; where "date-time" is as defined in RFC 5322 [4]
; but the "obs-" forms, especially two-digit
; years, are prohibited in SMTP and MUST NOT be used.
...

chrj/smtpd