For anyone else struggling with a similar issue, here's the code I used to add the time in a duration field to the start date/time and then populate the end date/time via a before insert/update business rule.
Converting the duration field to a GlideDateTime and then using the .getValue() function was crucial. Next you have to use the getNumericValue to get the time in milliseconds. From there you can manipulate the integer values from there and hopefully get what you're looking for...
(function executeRule(current, previous /*null when async*/ ) {
// Add your code here
// calculates the end time of an activity based on the start time and duration
var startTime = new GlideDateTime(current.variables.access_start.getValue()); // getting start time
var dur = new GlideDateTime(current.variables.how_long.getValue()); // getting duration and converting to GlideDateTime
dur = dur.getNumericValue()/1000; // calculating the total duration in seconds
//gs.addInfoMessage(dur);
startTime.addSeconds(dur); // add the seconds to the start time to calculate end time
current.variables.access_end = startTime.getValue(); // set the end time
gs.addInfoMessage("End time automatically adjusted based on start time and duration");
})(current, previous);
