2.1.3.2.7 Sent Time
To generate a Date header, clients set the value of the PidTagClientSubmitTime property ([MS-OXOMSG] section 2.2.3.11). The property value is expressed in Coordinated Universal Time (UTC).
MIME writers copy the value of the PidTagClientSubmitTime property to the Date header, formatting it as specified by [RFC2822] section 3.3. MIME writers SHOULD include hours, minutes, and seconds in the generated Date header value. MIME writers MAY convert the date and time value from UTC to another time zone of their choice.
If no value is specified for the PidTagClientSubmitTime property when a message is submitted to SMTP, MIME writers SHOULD generate a Date header with a value of the current time.
When generating a TNEF message, if the PidTagClientSubmitTime property is present on the Message object, MIME writers SHOULD<35> copy the value of the PidTagClientSubmitTime property to the attDateSent attribute, as specified in [MS-OXTNEF] section 2.1.3.3.4, and the attMsgProps attribute, as specified in [MS-OXTNEF] section 2.1.3.3.21.