If the PHP mail() function is allowed to generate the "Date:" header field, the sign of the timezone is inverted.

The system "timezone" is the number of minutes difference between UTC and local time (EST= +300).  In the Date: header, the timezone is formatted as "HHMM" and is the difference between local time and UTC (reversed difference, EST= -0500).

In win32/sendmail.c, the sign of the system "timezone" variable is copied to the Date: header without being inverted.

Also, there should be a sign even if the timezone offset is 0.  Therefore in sprintf() that generates the "Date:" header in the PostHeader() function, the timezone test should read:
      (_timezone > 0) ? "-" : "+",

This problem was also in the PHP3 source.

Regard, -Ron-