Expression manager: add PHP function strtotime

(Implemented T White's suggestions in this new version.)

facilitates the calculation with dates/times in expression manager. E.g.
calculation of difference in days between two dates entered in previous
questions of the survey

The function makes life much easier. Instead of
{mktime(0,0,0,substr(date1,3,2),substr(date1,0,2),substr(date1,6,4))}
one can just use {strtotime(date)}.

moreover, strtotime can deal with a lot of different date/time formats.
So, things like (strtotime(2013/12/23)-strtotime(1 Apr 1976))/3600/24 to calculate the number of days between two dates is easily done.