The attached patch improves the Mdocdate code and updates the
documentation to match.

 - in a2time():

   Skip past "$Mdocdate: " if it exists instead of relying on the caller
   to specify it in the format string.

   Accept trailing text after a successful match (e.g. "$").

   If the first character after a successful match is 'Z', use UTC
   instead of the local time zone.

 - in time2a():

   Use a sufficiently large stack buffer and mandoc_strdup() instead of
   trying to guess at the correct length.

 - in mandoc_normdate():

   Fix several logic errors (inverted tests), add a test for the
   Subversion %d format, and simplify the code.

 - in the man page:

   Document how to use Mdocdate with Subversion.

Given the following test cases:

% cat tests
February 7 2036
February  7 2036
February 07 2036
February 7, 2036
February  7, 2036
February 07, 2036
Feb 7 2036
Feb  7 2036
Feb 07 2036
Feb 7, 2036
Feb  7, 2036
Feb 07, 2036
2036-02-07
2036-02-7
2036-2-07
2036-2-7
$Mdocdate: February 7 2036 $
$Mdocdate: 2036-02-07 06:28:16Z $

The current code (1.13.3) produces the following output:

% while read d ; do echo ".Dd $d" | mandoc | tail -1 ; done <tests
                                February 7 2036
                                February 7 2036
                               February 07 2036
                               February 7, 2036
                               February 7, 2036
                               February 7, 2036
                                  Feb 7 2036
                                  Feb 7 2036
                                  Feb 07 2036
                               February 7, 2036
                               February 7, 2036
                               February 7, 2036
                                  2036-02-07
                                   2036-02-7
                                   2036-2-07
                                   2036-2-7
                               February 7, 2036
                       $Mdocdate: 2036-02-07 06:28:16Z $

My version produces the same output for all cases:

% while read d ; do echo ".Dd $d" | ./mandoc | tail -1 ; done <tests | uniq
                               February 7, 2036

DES
--
Dag-Erling Smørgrav - des@des.no