Loop table to find value

Hello,

Could someone please help me with a kind of "Vlookup" script for my form. I've searched the forum and google all day but cant find anything usefull (at least something understandable for my basic javascript knowladge ).

The basic idea is this:

1) On the form there will be a date field.

2) After a date has been entered a script schould loop a tables first column and find a maching month and year (for example if I input 26.03.2013. then it schould find 01.03.2013.). When a maching date has been found then it schould return a value from the tables second column.

Thanks in advance!