function isValidDate(dateStr) {
	// Date validation function courtesty of Sandeep V. Tamhankar (stamhankar@hotmail.com)
	// Checks for the following valid date formats:
	// MM/DD/YY   MM/DD/YYYY   MM-DD-YY   MM-DD-YYYY

	var datePat = /^(\d{1,2})(\/|-)(\d{1,2})\2(\d{4})$/; // requires 4 digit year
	var matchArray = dateStr.match(datePat); // is the format ok?

	if (matchArray == null) {
		alert("Date is not in a valid format.")
		return false;
	}

	month = matchArray[1]; // parse date into variables
	day = matchArray[3];
	year = matchArray[4];
	if (month < 1 || month > 12) { // check month range
		alert("Month must be between 1 and 12.");
		return false;
	}
	if (day < 1 || day > 31) {
		alert("Day must be between 1 and 31.");
		return false;
	}
	if ((month==4 || month==6 || month==9 || month==11) && day==31) {
		alert("Month "+month+" doesn't have 31 days!")
		return false;
	}
	if (month == 2) { // check for february 29th
		var isleap = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
		if (day>29 || (day==29 && !isleap)) {
			alert("February " + year + " doesn't have " + day + " days!");
			return false;
		}
	}
	return true;
}


function dispDate(dateObj) {
	month = dateObj.getMonth()+1;
	month = (month < 10) ? "0" + month : month;

	day   = dateObj.getDate();
	day = (day < 10) ? "0" + day : day;

	year  = dateObj.getYear();
	if (year < 2000) year += 1900;

	return (month + "/" + day + "/" + year);
}


function pregnancyCalc(pregform) {
	menstrual = new Date(); // creates new date objects
	ovulation = new Date();
	duedate = new Date();
	today = new Date();
	cycle = 0, luteal = 0; // sets variables to invalid state ==> 0

	if (isValidDate(pregform.menstrual.value)) { // Validates menstual date 
		menstrualinput = new Date(pregform.menstrual.value);
		menstrual.setTime(menstrualinput.getTime())
	}
	else return false; // otherwise exits

	cycle = (pregform.cycle.value == "" ? 28 : pregform.cycle.value); // defaults to 28
	// validates cycle range, from 22 to 45
	if (pregform.cycle.value != "" && (pregform.cycle.value < 22 || pregform.cycle.value > 45)) {
		alert("Your cycle length is either too short or too long for \n" + "calculations to be very accurate!  We will still try to \n" + "complete the calculation with the figure you entered. ");
	}

	luteal = (pregform.luteal.value == "" ? 14 : pregform.luteal.value); // defaults to 14
	// validates luteal range, from 9 to 16
	if (pregform.luteal.value != "" && (pregform.luteal.value < 9 || pregform.luteal.value > 16)) {
		alert("Your luteal phase length is either too short or too long for \n"	+ "calculations to be very accurate!  We will still try to complete \n" + "the calculation with the figure you entered. ");
	}

	// sets ovulation date to menstrual date + cycle days - luteal days
	// the '*86400000' is necessary because date objects track time
	// in milliseconds;  86400000 milliseconds equals one day
	ovulation.setTime(menstrual.getTime() + (cycle*86400000) - (luteal*86400000));
	pregform.conception.value = dispDate(ovulation);

	// sets due date to ovulation date plus 266 days
	duedate.setTime(ovulation.getTime() + 266*86400000);
	pregform.duedate.value = dispDate(duedate);

	// sets fetal age to 14 + 266 (pregnancy time) - time left
	var fetalage = 14 + 266 - ((duedate - today) / 86400000);
	weeks = parseInt(fetalage / 7); // sets weeks to whole number of weeks
	days = Math.floor(fetalage % 7); // sets days to the whole number remainder

	// fetal age message, automatically includes 's' on week and day if necessary
	fetalage = weeks + " week" + (weeks > 1 ? "s" : "") + ", " + days + " days";
	pregform.fetalage.value = fetalage;

	return false; // form should never submit, returns false
}