Convert non-relative date.js-like string to a js Date object

DateGenerator.js

// see also https://github.com/MatthewMueller/date/issues/57

// usage: var someDateObj = DateGenerator("Jan 3 at 4 pm").dt;

function DateGenerator(str) {
    function lookupMonth(str) {
        var monthsFull = [ 'January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December' ],
            monthsShort = [ 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec' ],
            result = -1
        ;

        result = (monthsFull.indexOf(str) > -1)
                ? monthsFull.indexOf(str)
                : (monthsShort.indexOf(str) > -1)
                    ? monthsShort.indexOf(str)
                    : -1
        ;

        if (result >= 0) result++; // make 1-based index (human)

        return result;
    }

    function generateFromString(str) {
        var parts = str.split(' '),

            // extract string parts:
            //   expects a string with structure "MMM DD at HH [am|pm]"
            //   ie. "Jan 1 at 3 pm"
            month = parts[0],
            d     = parts[1],
            at    = parts[2], //fixme: detect (optional) year
            hour  = parseInt(parts[3], 10),
            ampm  = parts[4],

            _date = new Date(), // today
            yyyy  = _date.getFullYear(),
            _mm   = _date.getMonth() + 1,
            mm    = lookupMonth(month),
            date1,
            date2,
            hourOffset
        ;

        //fixme: if requested month is less than current month, assume next year.
        if (mm < _mm) {
            yyyy++;
        }

        date1 = new Date(yyyy + '-' + mm + '-' + d + ' 00:00:00'); // 12:00am on specified date

        if (ampm.toLowerCase() == 'am' && hour == 12) hour = 0;
        if (ampm.toLowerCase() == 'pm' && hour < 12) hour += 12;

        hourOffset = ((60000*60) * hour);
        date2 = new Date(date1.valueOf() + hourOffset);
        return date2;
    }

    var dt = new Date();

    if (str) {
        dt = generateFromString(str);
    }

    return {
        str: str,
        dt: dt,
        lookupMonth: lookupMonth,
        generateFromString: generateFromString
    };
}