java - Adding minutes to Calendar object -

i attempting add amount of time calendar object. input coming time picker , output goes 2 strings.

    dateformat dateformat = new android.text.format.dateformat();      calendar starttimecalendar = calendar.getinstance();      //these 2 string this: "08:15am"     int prevstarthour = integer.parseint(starttime.gettext().subsequence(0, 2).tostring());     int prevstartminute = integer.parseint(starttime.gettext().subsequence(3, 5).tostring());     int prevendhour = integer.parseint(endtime.gettext().subsequence(0, 2).tostring());     int prevendminute = integer.parseint(endtime.gettext().subsequence(3, 5).tostring());      //get difference in hours between 2 strings     int hourchange = math.abs(prevendhour - prevstarthour);     //get difference in minutes between 2 strings     int minutechange = math.abs(prevendminute - prevstartminute);      //set date on first label     starttime.settext(dateformat.format("hh:mmaa", starttimecalendar).tostring());     //add difference in hours calendar     starttimecalendar.add(calendar.hour_of_day, hourchange);     //add difference in minutes calendar     starttimecalendar.add(calendar.minute, minutechange);     //set date on second label     endtime.settext(dateformat.format("hh:mmaa", starttimecalendar).tostring()); 

now odd part... hours add fine , dandy while minutes decide stop working halfway through. instead of adding expected time (be 15, 30 or 45min) adds 30min every time.

change hh:mmaa hh:mmaa (link).