First of all:
This convertor is bull****. Look at it! Just LOOK at the silly thing!
I once wrote a beautiful function to convert dd/mm/yyyy dates to indices, so you could subtract the indices and determine the number of dates between dates.
If you want seconds: Feed the dates into Date() objects (remember to allow for the funny month numbering!), then subtract these Date() objects to get milliseconds. Divide this number by 1000 to get seconds (NB: this number will probably not be an integer unless you round it).
I'm not through telling you about the Date() object. If you give it a date as a string, it will evaluate it with layman's month numbering: new Date("01/01/2006") gives you New Year's Day 2006. But if you give it a date as numbers, the eccentric month numbering is used: new Date(2006,1,1) gives you Feb. 1, 2006.