To get the current date and time to minutes, you could divide the milliseconds:
var theMinutes = new Date().getTime() / (1000 * 60);
To get the input and convert it to minutes, you could use a Date object, and divide the milliseconds:
var someDateTime = document.getElementById("someDateTime").value;
var someParts = someDateTime.split(" ");
var theDate = someParts[0];
var theTime = someParts[1];
var dateParts = theDate.split("/");
var timeParts = theTime.split(":");
var theDay = dateParts[0];
var theMonth = dateParts[1];
var theYear = dateParts[2];
var theHours = timeParts[0];
var theMinutes = timeParts[1];
var someDateTimeObject = new Date(theYear, theMonth, theDay, theHours, theMinutes, 0 );
var someDateTimeObjectMillis = someDateTimeObject.getTime();
var someDateTimeObjectMinutes = someDateTimeObjectMillis / (1000 * 60);