$(document).ready(function() {

        var containingString = '.this.that.';
        subString = '.';

        var index = GetIndexOfSubstring(containingString, subString, 2);

    });

    function GetIndexOfSubstring(containingString, subString, occurrenceNumberOfSubString)
    {
        tokens = containingString.split(subString);
        var numberOfSubStrings = tokens.length - 1;

        if (occurrenceNumberOfSubString > numberOfSubStrings)
            return "Error";

        var index = 0 + occurrenceNumberOfSubString - 1;

        for(var c = 0; c < occurrenceNumberOfSubString; c++)
        {
            var i = containingString.indexOf(subString);
            var sub = containingString.substr(i + 1);
            containingString = sub;
            index += i;
        }

        return index;
    }

function GetSubstringIndex(str, substring, n) {
    var times = 0, index = null;

    while (times < n && index !== -1) {
        index = str.indexOf(substring, index+1);
        times++;
    }

    return index;
}