Try this
function validate(dt1, dt2)
{
var jdt1 = Date.parse('20 Aug 2000 ' + dt1);
var jdt2 = Date.parse('20 Aug 2000 ' + dt2);
var diffMs;
var diffMins
if (isNaN(jdt1))
{
alert('invalid start time');
return false;
}
if (isNaN(jdt2))
{
alert('invalid end time');
return false;
}
if (jdt1 > jdt2)
{
diffMs = (jdt1 - jdt2);
diffMins = Math.round(((diffMs % 86400000) % 3600000) / 60000); // minutes
alert('start is greater by ' + diffMins + ' minutes');
}
else
{
diffMs = (jdt2 - jdt1);
diffMins = Math.round(((diffMs % 86400000) % 3600000) / 60000); // minutes
alert('start is less equal by ' + diffMins + ' minutes');
}
return true;
}