Get the amount of days, weeks, months, years between dates with JavaScript (IE7+)

gistfile1.js

var $date = function() {
 
    function getDaysInMonths(m, y) {
        return /8|3|5|10/.test(--m) ? 30 : m == 1 ? (!(y % 4) && y % 100) || !(y % 400) ? 29 : 28 : 31;
    }

    function isLeapYear(y) {
	return getDaysInMonths(2, y) === 29;
    }

    function getTodaysDay() {
	return new Date().getDate();
    }

    function getTodaysMonth() {
	return new Date().getMonth() + 1;
    }

    function getTodaysYear() {
	return new Date().getFullYear();
    }

    function getDaysBetweenTwoDates(d1, m1, y1, d2, m2, y2) {

	var diff = getDifferenceInYears(y2, y1), total = 0;	

	if(!diff) {
	    return getDaysBetweenTwoDatesInSameYear(d1, m1, d2, m2, y1);
	}
	else {

	    if(diff === 1) {	

		return getDaysBetweenTwoDatesInSameYear(d1, m1, 31, 12, y1) +
			getDaysBetweenTwoDatesInSameYear(1, 1, d2, m2, y2);

	    }
	    else {

	        total += getDaysInYears(y1 + 1, y2);			
	
	        total += getDaysBetweenTwoDatesInSameYear(d1, m1, 31, 12, y1) +
			getDaysBetweenTwoDatesInSameYear(1, 1, d2, m2, y2);

	        return total;	

	    }

        }

    }

    function getDaysBetweenTwoDatesInSameYear(d1, m1, d2, m2, y) {

	var total = 0, leapYear;

	if(m1 === m2) {
	    return d2 - d1;
	}
	else {

	    leapYear = isLeapYear(y) ? true : false;

	    if(m2 - m1 === 1) {				
		return getDaysRemainingFromMonth(d1, m1, leapYear) + d2;				
	    } 
	    else {

		total+= getDaysRemainingFromMonth(d1, m1, leapYear) + d2;
		total += getDaysInMonthsBetween(m1 + 1, m2, y);

		return total;

	    }

	}

    }

    function getDaysInMonthsBetween(startMonth, stopMonth, y) {

	for(var total = 0; startMonth < stopMonth; startMonth++) {
	    total += getDaysInMonths(startMonth, y);				
	}

	return total;

    }

    function getDaysInYears(startYear, stopYear) {

	for(var total = 0; startYear < stopYear; startYear++) {
	    total += getDaysBetweenTwoDatesInSameYear(1, 1, 31, 12, startYear);
	}

	return total;

    }

    function getDifferenceInYears(y1, y2) {
	return y1 - y2;
    }

    function getDaysRemainingFromMonth(d, m, ly) {

	var y = ly ? 2016 : 2015;
	d = d === 1 ? 0 : d;
	return getDaysInMonths(m, y) - d;

    }

    function getWeeksFromDays(d) {
	return Number((d / 7).toFixed(1));
    }

    function getHoursFromDays(d) {
	return d * 24;
    }

    function getMonthsFromDays(d) {
 	return Number((getYearsFromDays(d) * 12).toFixed(2));
    }

    function getYearsFromDays(d) {
	return Number((d / 365).toFixed(2));
    }	

    function getDaysFromDate(d, m, y) {
	return getDaysBetweenTwoDates(d, m, y, getTodaysDay(), getTodaysMonth(), getTodaysYear());
    }

    function getDaysToDate(d, m, y) {
	return getDaysBetweenTwoDates(getTodaysDay(), getTodaysMonth(), getTodaysYear(), d, m, y);
    }

    function daysOrDate(args, fn) {
		
	if(args.length > 1) {
	    return fn(args[0], args[1], args[2]);
	}
	else {
	    return args[0];
	}

    }

    return {

	getDaysSinceDate : function(d, m, y) {
	    return getDaysFromDate(d, m, y);
	},
	getDaysUntilDate : function(d, m, y) {
	    return getDaysToDate(d, m, y);
	},
	getWeeksSinceDate : function(d, m, y) {				
	    return getWeeksFromDays(daysOrDate(arguments, getDaysFromDate));
	},
	getMonthsSinceDate : function(d, m, y) {
	    return getMonthsFromDays(daysOrDate(arguments, getDaysFromDate));
	},
	getYearsSinceDate : function(d, m, y) {
	    return getYearsFromDays(daysOrDate(arguments, getDaysFromDate));
	},
	getWeeksUntilDate : function(d, m, y) {
	    return getWeeksFromDays(getDaysToDate(d, m, y));
	},
	getMonthsUntilDate : function(d, m, y) {
	    return getMonthsFromDays(getDaysToDate(d, m, y));
	},
	getYearsUntilDate : function(d, m, y) {
	    return getYearsFromDays(getDaysToDate(d, m, y));
	},
	getDaysBetweenTwoDates : function(d1, m1, y1, d2, m2, y2) {
	    return getDaysBetweenTwoDates(d1, m1, y1, d2, m2, y2);
	}

    }		

};

// returns the amount of days, weeks, months or years that have passed since a date (d,m,y)
// or the days, weeks, months, years until a date
// or the days, weeks, months, years between two dates*
console.log($date().getDaysSinceDate(21, 11, 1979));	
console.log($date().getWeeksSinceDate(21, 11, 1979));
console.log($date().getMonthsSinceDate(21, 11, 1979));
console.log($date().getYearsSinceDate(21, 11, 1979));

console.log($date().getDaysUntilDate(21, 11, 2015));
console.log($date().getWeeksUntilDate(21, 11, 2015));
console.log($date().getMonthsUntilDate(21, 11, 2015));
console.log($date().getYearsUntilDate(21, 11, 2015));

console.log($date().getDaysBetweenTwoDates(1, 1, 2015, 13, 3, 2015)); // 72
console.log($date().getDaysBetweenTwoDates(1, 1, 2013, 13, 3, 2015)); // 802
console.log($date().getDaysBetweenTwoDates(1, 2, 2015, 13, 2, 2015)); // 12
console.log($date().getDaysBetweenTwoDates(21, 1, 2015, 7, 2, 2015)); // 17
console.log($date().getDaysBetweenTwoDates(1, 1, 2015, 1, 3, 2015)); // 60
console.log($date().getDaysBetweenTwoDates(21, 1, 2015, 1, 3, 2015)); // 39
console.log($date().getDaysBetweenTwoDates(21, 11, 2014, 13, 2, 2015)); // 84
console.log($date().getDaysBetweenTwoDates(31, 3, 2013, 13, 2, 2015)); // 684
console.log($date().getDaysBetweenTwoDates(21, 11, 1979, 15, 2, 2015)); // 12870

// *find the total amount of weeks, months, years between two dates -
// override the getWeeks, getMonths and getYearsSince methods by passing in getDaysBetweenTwoDates as a single argument
console.log($date().getMonthsSinceDate($date().getDaysBetweenTwoDates(15, 2, 2013, 15, 2, 2015))); // 24