Best way to subtract a date from one another?

So basically I have two dates and I need to subtract one from the other. It's for when the player is away and then they come back. I need to take the new time and time they started from and figure out how much time has passed. None of the code is tested, I'm just trying to find the most efficient and possibly best way to do this. Here is what I have so far though:

Code:

                for(UUID users : timeBeenAwayStart.keySet()) {
                    if(!afkManager.isAfk(Bukkit.getPlayer(users))) {
                        //No longer AFK, add their start time
                        String timeStamp = new SimpleDateFormat("MM:dd:HH:mm:ss").format(Calendar.getInstance().getTime());
                        String addToTime = "";
                        try {
                            String nowTime[] = timeStamp.split(":");
                            String startTime[] = timeBeenAwayStart.get(users).split(":");
                            /* Trial 1 Test? */
                            int monthsBetween = Integer.parseInt(nowTime[0]) - Integer.parseInt(startTime[0]);
                            if(!(monthsBetween > 0)) {
                                monthsBetween = 0;
                            }
                            int daysBetween = Integer.parseInt(nowTime[1]) - Integer.parseInt(startTime[1]);
                            if(!(daysBetween > 0)) {
                                daysBetween = 0;
                            }
                            int hoursBetween = Integer.parseInt(nowTime[2]) - Integer.parseInt(startTime[2]);
                            if(!(hoursBetween > 0)) {
                                hoursBetween = 0;
                            }
                            int minutesBetween = Integer.parseInt(nowTime[3]) - Integer.parseInt(startTime[3]);
                            if(!(minutesBetween > 0)) {
                                minutesBetween = 0;
                            }
                            int secondsBetween = Integer.parseInt(nowTime[4]) - Integer.parseInt(startTime[4]);
                            if(!(secondsBetween > 0)) {
                                secondsBetween = 0;
                            }
                            if(!(monthsBetween > 9)) {
                                //It needs a zero in front
                                addToTime += "0" + String.valueOf(monthsBetween) + ":";
                            } else {
                                addToTime += String.valueOf(monthsBetween) + ":";
                            }
                            if(!(daysBetween > 9)) {
                                //It needs a zero in front
                                addToTime += "0" + String.valueOf(daysBetween) + ":";
                            } else {
                                addToTime += String.valueOf(daysBetween) + ":";
                            }
                            if(!(hoursBetween > 9)) {
                                //It needs a zero in front
                                addToTime += "0" + String.valueOf(hoursBetween) + ":";
                            } else {
                                addToTime += String.valueOf(hoursBetween) + ":";
                            }
                            if(!(minutesBetween > 9)) {
                                //It needs a zero in front
                                addToTime += "0" + String.valueOf(minutesBetween) + ":";
                            } else {
                                addToTime += String.valueOf(minutesBetween) + ":";
                            }
                            if(!(secondsBetween > 9)) {
                                //It needs a zero in front
                                addToTime += "0" + String.valueOf(secondsBetween);
                            } else {
                                addToTime += String.valueOf(secondsBetween);
                            }
                            if(referredPlayerInstances.containsKey(users)) {
                                //They have an instance
                                if(!addToTime.equals("")) {
                                    referredPlayerInstances.get(users).addToEndTime(addToTime);
                                }
                            }
                            /**/
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                }

 

Is there a way I can convert the milliseconds back to the date format I need?

 

Code:

for(UUID users : timeBeenAwayStart.keySet()) {
                    if(!afkManager.isAfk(Bukkit.getPlayer(users))) {
                        //No longer AFK, add their start time
                        String timeStamp = new SimpleDateFormat("dd:HH:mm:ss").format(Calendar.getInstance().getTime());
                        String startedAfkDate = timeBeenAwayStart.get(users);
                        String addToTime = "";
                        try {
                            Date currentTime = new SimpleDateFormat("dd:HH:mm:ss").parse(timeStamp);
                            Date afkStartDate = new SimpleDateFormat("dd:HH:mm:ss").parse(startedAfkDate); //This [1]
                            String nowTime[] = timeStamp.split(":");
                            String startTime[] = timeBeenAwayStart.get(users).split(":");
                            long currentDay = TimeUnit.DAYS.toDays(System.currentTimeMillis());
                            long currentHour = TimeUnit.HOURS.toHours(System.currentTimeMillis());
                            long currentMinute = TimeUnit.MINUTES.toMinutes(System.currentTimeMillis());
                            long currentSeconds = TimeUnit.SECONDS.toSeconds(System.currentTimeMillis());
                            // How to get currentMillis of their startDate [1]
                            /* Trial 1 Test? */
                            if(referredPlayerInstances.containsKey(users)) {
                                //They have an instance
                                if(!addToTime.equals("")) {
                                    referredPlayerInstances.get(users).addToEndTime(addToTime);
                                }
                            }
                            /**/
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                }

I think I fixed it and figured out an easier way!

Code:

for(UUID users : timeBeenAwayStart.keySet()) {
                    if(!afkManager.isAfk(Bukkit.getPlayer(users))) {
                        //No longer AFK, add their start time
                        long timeStartedAway = timeBeenAwayStart.get(users);
                        try {
                            /* Right now: */
                            long currentMillis = System.currentTimeMillis();
                            /* Trial 1 Test? */
                            if(referredPlayerInstances.containsKey(users)) {
                                //They have an instance
                                /* ChangedTimeMillis */
                                long changedTimeMillis = currentMillis - timeStartedAway;
                                //String newTime = new SimpleDateFormat("mm:ss:SSS").format(new Date(changedTimeMillis));
                                referredPlayerInstances.get(users).addToEndTime(changedTimeMillis);
                            }
                            /**/
                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                    }
                }

In my RefferedPlayerInstances

Code:

    public void addToEndTime(long timeMillis) {
        this.endTimeMillis += timeMillis;
        this.endTime = new SimpleDateFormat("mm:ss:SSS").format(new Date(this.endTimeMillis));
    }

EDIT by Moderator: merged posts, please use the edit button instead of double posting.