Change datefield

Hi all,
I'm starting to learn Javascript, but I desperately need to adjust a piece of code right now:

I have this code which trims the date from a 'enddate' box.
What I want now is to adjust this code so that it returns 'startdate' 17:00 h

So: I choose startdate = 15-2-2017 08:00
or even better: I choose startdate 15-2-2017, the code fills the time: 08:00
enddate auto fills with: 15-2-2017 17:00

Can anyone make something out of this?
Thanks!

var startDate = $('.pss_fieldname_begindatetime input').val().substring(0,$('.pss_fieldname_begind atetime input').val().indexOf(" ")).trim();

var dateTimeInput = $(this).find("input");
dateTimeInput.on("focus", function() {
setTimeout(function(){
dateTimeInput.datetimepicker({
datepicker:false
});
}, 500);
});

var isDisallowedKeycode = function(keycode){
return (
((keycode >= 48) && (keycode <= 90)) || ((keycode >= 96) && (keycode <= 111)) || ((keycode >= 186) && (keycode <= 22))
);
}

dateTimeInput.on("keydown", function(event){
if (isDisallowedKeycode(event.which)){
return false;
}
});