The $d macro holds the current date and time. $d is given its value at the same time $b is defined. The only difference between the two is that $b contains the date in RFC822 format, whereas $d contains the same date in Unix ctime(3) format.

The form of a date in ctime(3) format is generally:^[16]

^[16] The format produced by ctime(3) varies depending on your location.

Sun Dec 26 01:03:52 2003\n\

When sendmail stores this form of date into $d, it converts the trailing newline (the \n) into a zero, thus stripping the newline from the date.