Oh, here is some output of Date::_parse directly:

$ ruby -w -rdate -e 'p Date._parse("12/31/75 21:28:35", true)'
{:sec=>35, :year=>1975, :hour=>21, :mday=>31, :min=>28, :mon=>12}
$ ruby19 -w -rdate -e 'p Date._parse("12/31/75 21:28:35", true)'
{:hour=>21, :min=>28, :sec=>35, :year=>2012, :mon=>31, :mday=>75}

month == 31? mday == 75? Really??

Cheers,
Jim