This is some code I use to have a user select one
datetimepicker, set the datetime, and have the
other datetimepicker add One Minute to that time.
I needed this for a custom medication control....
Anyway, thought it might help someone else since I could
not find the answer any where online... 
(at least not a complete answer)
Keep in mind that the 60000 added, adds one minute.
(60 * 1000 milliseconds)
$('.frdtPicker').datetimepicker({
        onClose: function(dateText, inst) {
            var endDateTextBox = $('.todtPicker');
            if (endDateTextBox.val() != '') {
                var testStartDate = new Date(dateText);
                var testEndDate = new Date(endDateTextBox.val());
                if (testStartDate > testEndDate) {
                    var testStartDate = new Date(dateText).getTime() + 60000;
                    var testStartDate2 = new Date(testStartDate); 
                    endDateTextBox.datetimepicker('setDate', (new Date(testStartDate2)));
                }
            }
            else {
                var testStartDate = new Date(dateText).getTime() + 60000;
                var testStartDate2 = new Date(testStartDate);
                endDateTextBox.datetimepicker('setDate', (new Date(testStartDate2)));
            }
            $('.frdtPicker').val(dateText); //endDateTextBox.val());
        },
        onSelect: function(selectedDateTime) {
            var start = $(this).datetimepicker('getDate');
            $('.todtPicker').datetimepicker('option', 'minDate', new Date(start.getTime()));
        }
    });