function isValidDate(dateStr) {

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(dateStr + " 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 isValidTime(timeStr) {

var timePat = /^(\d{1,2}):(\d{2})(:(\d{2}))?(\s?(AM|am|PM|pm))?$/;

var matchArray = timeStr.match(timePat);
if (matchArray == null) {
alert("Time is not in a valid format.");
return false;
}
hour = matchArray[1];
minute = matchArray[2];
second = matchArray[4];
ampm = matchArray[6];

if (second=="") { second = null; }
if (ampm=="") { ampm = null }

if (hour < 0  || hour > 23) {
alert("Hour must be between 1 and 12. (or 0 and 23 for military time)");
return false;
}
if (hour <= 12 && ampm == null) {
if (confirm("Please indicate which time format you are using.  OK = Standard Time, CANCEL = Military Time")) {
alert("You must specify AM or PM.");
return false;
   }
}
if  (hour > 12 && ampm != null) {
alert("You can't specify AM or PM for military time.");
return false;
}
if (minute < 0 || minute > 59) {
alert ("Minute must be between 0 and 59.");
return false;
}
if (second != null && (second < 0 || second > 59)) {
alert ("Second must be between 0 and 59.");
return false;
}
return true;
}

function dateDiff(dateform) {
date1 = new Date();
date2 = new Date();
diff  = new Date();

if (isValidDate(dateform.firstdate.value) && isValidTime(dateform.firsttime.value)) { // Validates first date 
date1temp = new Date(dateform.firstdate.value + " " + dateform.firsttime.value);
date1.setTime(date1temp.getTime());
}
else return false; // otherwise exits

if (isValidDate(dateform.seconddate.value) && isValidTime(dateform.secondtime.value)) { // Validates second date 
date2temp = new Date(dateform.seconddate.value + " " + dateform.secondtime.value);
date2.setTime(date2temp.getTime());
}
else return false; // otherwise exits

// sets difference date to difference of first date and second date

diff.setTime(Math.abs(date1.getTime() - date2.getTime()));

timediff = diff.getTime();

weeks = Math.floor(timediff / (1000 * 60 * 60 * 24 * 7));
timediff -= weeks * (1000 * 60 * 60 * 24 * 7);

days = Math.floor(timediff / (1000 * 60 * 60 * 24)); 
timediff -= days * (1000 * 60 * 60 * 24);

hours = Math.floor(timediff / (1000 * 60 * 60)); 
timediff -= hours * (1000 * 60 * 60);

mins = Math.floor(timediff / (1000 * 60)); 
timediff -= mins * (1000 * 60);

secs = Math.floor(timediff / 1000); 
timediff -= secs * 1000;

dateform.difference.value = weeks + " weeks, " + days + " days, " + hours + " hours, " + mins + " minutes, and " + secs + " seconds";

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