SpamAssassin Rule: DATE_IN_PAST_12_24
Standard description: Date: is 12 to 24 hours before Received: date
The Date: header is checked against the timestamps in the Received: header lines. If the clock generating the Date: header is accurate then the mail was generated 12 to 24 hours before being received by one of the relays. This is unlikely with a normal mail client.
The default scores for this rule can be found in the online list of tests.