Java:

public static String timeConversion(String str) {

  String time12HourFormat = str;

    SimpleDateFormat inputFormat = new SimpleDateFormat("hh:mm:ssa");

    SimpleDateFormat outputFormat = new SimpleDateFormat("HH:mm:ss");

    try{

        Date date = inputFormat.parse(time12HourFormat);

        String time24HourFormat = outputFormat.format(date); 

        return time24HourFormat;

    } catch (ParseException e) {    
        e.printStackTrace();                    
    }
    return "";
   }