java - Add time to 9-hours days -

i'm working on basic project manages tasks. working days 8:00 17:00. task has start time , estimated duration. estimated duration in minutes. want calculate end time.

i've figured out how add days start time far precise. divide estimated duration 540 (= 9 hours) know how many days there approximately are. want able calculate end date precisly.

i can't can't add minutes calendar because calendar uses 24-hours days instead of 9-hours days. example if start time 2015-04-26 16:00:00.0 , add 180 minutes (3 hours) end time 2015-04-27 10:00:00.0. how done? have far:

public timespan calculateestimatedduration()  {         simpledateformat format = new simpledateformat("yyyy-mm-dd hh:mm:ss.s");          //divide 540 because there 9 hours in work day         long workingdays = math.round((this.getestimatedduration()/540));          //convert calendar add estimated working days start date         date startdate =  this.getplannedstarttime();         calendar cal = calendar.getinstance();         cal.settime(startdate);         cal.add(calendar.date,(int) workingdays);         string endtime = format.format(cal.gettime());         date plannedendtime = format.parse(endtime);          return new timespan(this.getplannedstarttime(),plannedendtime);     } 

i've found solution using modulo operator, explanation in commentary:

public timespan addestimatedtotime(date time) {     simpledateformat format = new simpledateformat("yyyy-mm-dd hh:mm:ss.s");      //first days     long workingdays =  this.getestimatedduration() / 540;     //then remaining hours , minutes     long remainderdays = this.getestimatedduration() % 540;     //then calculate hours     long workinghours = remainderdays/60;     //and remaining minutes     long workingminutes = remainderdays % 60;      //convert calendar add estimated time given time     calendar cal = calendar.getinstance();     cal.settime(time);     cal.add(calendar.date,(int) workingdays);     cal.add(calendar.hour , (int) workinghours);     cal.add(calendar.minute, (int) workingminutes);      //format date     string endtime = format.format(cal.gettime());     date plannedendtime = format.parse(endtime);      return new timespan(time,plannedendtime); }