var duration = cur_frm.doc.duration;
var postingTime = cur_frm.doc.posting_time;
var hour=0;
var minute=0;
var second=0;
var splitDuration= duration.split(':');
var splitPostingTime= posting_time.split(':');
hour = parseInt(splitDuration[0])+parseInt(splitPostingTime[0]);
minute = parseInt(splitDuration[1])+parseInt(splitPostingTime[1]);
hour = hour + minute/60;
minute = minute%60;
second = parseInt(splitDuration[2])+parseInt(splitPostingTime[2]);
minute = minute + second/60;
second = second%60;
cur_frm.set_value("end_time",hour+':'+minute+':'+second);
cur_frm.refresh_fields("end_time")
hope you find it useful