Consistent, Exact Aniversary Counter

timesince.js

var YEAR = 31536000000;
var DAY = 86400000;
var HOUR = 3600000;
var MINUTE = 60000;
var SECOND = 1000;

/**
 * Takes two date objects and returns the time difference of the present date 
 * since the past date.
 * @param {Date} past The past date
 * @param {Date} present The presetn date
 * @returns {years: number, months: number, hours: number,  minutes: number, seconds: number, milliseconds: number}
 */
function timeSince(past, present) {
	var pastMonthStart = new Date(past.getFullYear(), past.getMonth());
	var pastIntoMonth = past.getTime() - pastMonthStart.getTime();

	var monthStart = new Date(present.getFullYear(), present.getMonth());
	// milliseconds into the current month
	var milliseconds = present.getTime() - monthStart.getTime();

	// whole calendar months past
	var monthsPast = (present.getFullYear() - past.getFullYear()) * 12 + (present.getMonth() - past.getMonth() - 1);
	// another if gone past anniversary in the current month
	if (milliseconds > pastIntoMonth) {
		milliseconds -= pastIntoMonth;
		monthsPast++;
	}

	// a year is 12 months, whatever a month may be
	var years = Math.floor(monthsPast / 12);
	var months = monthsPast % 12;

	var days = Math.floor(milliseconds / DAY);
	milliseconds -= days * DAY;
	var hours = Math.floor(milliseconds / HOUR);
	milliseconds -= hours * HOUR;
	var minutes = Math.floor(milliseconds / MINUTE);
	milliseconds -= minutes * MINUTE;
	var seconds = Math.floor(milliseconds / SECOND);
	milliseconds -= seconds * SECOND;
	
	return {
		years: years, 
		months: months,
		hours: hours, 
		minutes: minutes,
		seconds: seconds,
		milliseconds: milliseconds
	};
}