I've started to notice more of these sorts of messages in my logcheck mails:

spamd[11313]: Second '70' out of range 0..59 at /usr/share/perl5/Mail/SpamAssassin/Util.pm line 429

I check the mail and, sure enough, the date header specifies a time with 70 seconds. Why should I spend resources running a mail through the full automated content analysis if the spammer can't be bothered even attempting to come up with a plausible date?

I've decided to reject based on malformed Date headers before the mail even gets passed to SpamAssassin. The following can be put in the acl_check_data file before any spam acl. The last check is for "obsolete" RFC822 style dates, which shouldn't be generated by any software written in the last five years - I'm just tagging these for now to assess the impact of blocking them.

deny  condition = ${if !def:h_date: {1}}
      message = Message SHOULD have Date header but does not
      log_message = acl_check_data: No Date header

# note: see RFC2822
# + some folded whitespace must implemented as a single space
# + loose checking on CFWS
# bug: will reject mails sent on a leap second, too bad
# bug: needs to be modified to work after 2099
deny    log_message = acl_check_data: RFC2822 Date invalid "$h_date:"
        message = Date header is invalid
        !condition = ${if match {$h_date:}{\N^\s*((Mon|Tue|Wed|Thu|Fri|Sat|Sun),)?\s*?[0-3]?[0-9] (Jan|Feb|Ma[ry]|Apr|Ju[nl]|Aug|Sep|Oct|Nov|Dec) (19|20)?[0-9]{2} [0-2][0-9](\:[0-5][0-9]){1,2} (?:[+-][01][0-9][0-5][0-9]|UT|[A-Z]{2,3}T|[A-Z])(\s(\(.*\))?)*$\N} {1}}
# note: as above, but requires 4 digit year and 4 digit timezone
warn    log_message = acl_check_data: RFC2822 Date format obsolete "$h_date:"
        message = X-RFC2822-Warning: Date format obsolete
        !condition = ${if match {$h_date:}{\N^\s*((Mon|Tue|Wed|Thu|Fri|Sat|Sun),)?\s*?[0-3]?[0-9] (Jan|Feb|Ma[ry]|Apr|Ju[nl]|Aug|Sep|Oct|Nov|Dec) 20[0-9]{2} [0-2][0-9](\:[0-5][0-9]){1,2} [+-][01][0-9][0-5][0-9](\s(\(.*\))?)*$\N} {1}}

Examples of the sorts of dates being rejected:

Date: Sat, 26 Aug 2006 11:23:55 -0200 EST
Date: , 26 Aug 2006 12:23:00 +0900
Date:  %CURRENT_DATE_TIME
Date: Sat, 26 Aug 2006 12:00:44 0000
Date: Lun, 28 Ago 2006 04:48:16
Date:  Wed, 30 Aug 2006 00:12:24 -05-30
Date: 29 Aug 2006 18:56:70 +0100
Date: Thu, 2 Jan 1997 13:21:00 -0060