Author: Tim Hunt
MDL-82687 formslib: only allow positive durations by default
http://git.moodle.org/gw?p=moodle.git;a=commit;h=fc19af4ce52a328505aaacd7642a7bc2c77c0744
http://github.com/moodle/moodle/commit/fc19af4ce52a328505aaacd7642a7bc2c77c0744
+ 4.5 en [err_positiveduration,core_form]