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Get Date and Time by Timezone; irrespective of machine's time

I am working on Spring MVC project. It is an quasi online system where each client will install our system (Tomcat n Mysql will get installed through an installer) on their machine. They will download the data by connecting their machine to internet. Once data get downloaded they can disconnect from internet.
By considering above scenario, we want to validate the system date n time is correct according to time zone. I have checked How to get local time of different time zones?, The code :
java.util.TimeZone tz = java.util.TimeZone.getTimeZone("IST");
java.util.Calendar c = java.util.Calendar.getInstance(tz);
System.out.println(c.get(java.util.Calendar.HOUR_OF_DAY)+":"+c.get(java.util.Calendar.MINUTE)+":"+c.get(java.util.Calendar.SECOND));
Give the same time of the system. I want something which will tell time according to the time zone. Same as like when we set time zone on OS clock it will automatically set the correct date and time according to that time zone.
Implementation like this:
Date accurateTimeZoneDate = //HERE I WANT SOMETHING TO GET DATE ACCORDING TO TIME ZONE.
Date machineCurrentDate = new Date();
if(accuratetimeZoneDate == machineCurrentDate)
{
//machine date and time zone date is correct.
}
else
{
//machine date and time zone date is NOT correct.
}
Update
We have tried this:
Daily it is mandatory to connect the system to internet so that application will ping to an central ntp and get the time and validate. once validation is successful then they can disconnect from internet. But in this case after validation they switch to some old date and use the expired content.

Ok if you want system time and date then you should try new Date().
it will give current date time with timezone.
Instead of using Calendar, you should use new Date()
System.out.println("Current Date And Time"+ new Date());

Modify date without modifying time

With JodaTime, without using the 'plus' or 'minus' functions and using the least lines of code, how can I set a new date without modifying the time?
My first attempt was to store the 'time' parts of the DateTime in separate ints using getHoursOfDay() and getMinutesOfHour() etc - then create a new DateTime with the required date and set the hours, minutes, and seconds back again. But this method is pretty clunky, and I was wondering if there was a less verbose method for doing this - ideally with just one line of code.
For example:
22/05/2013 13:40:02 >>>> 30/08/2014 13:40:02

Is JodaTime a must? Basic way to do this is
1. extract just time from timestamp.
2. add this to just date
long timestamp = System.currentTimeMillis(); //OK we have some timestamp
long justTime = timestamp % 1000 * 60 * 60 * 24;// just tiem contains just time part
long newTimestamp = getDateFromSomeSource();//now we have date from some source
justNewDate = newTimestamp - (newTimestamp % 1000 * 60 * 60 * 24);//extract just date
result = justNewDate + justTime;
Something like this.

Previously accepted answer were removed by moderator, as it contains only link to javadoc.
Here is edited version.
You could do it like this
DateTime myDate = ...
myDate.withDate(desiredYear, desiredMonth, desiredDayOfMonth);
JavaDoc is here: DateTime.withDate(int year, int month, int dayOfMonth)

use withFields like this:
new DateTime().withFields(new LocalDate(2000,1,1))
This will set all date time fields of the DateTime to those that are contained in the LocalDate - year, month and day in this case. This will work with any ReadablePartial implementation like YearMonth for example.

Dealing with saved epoch times when clocks change

I have an alarm clock app that uses the following two methods:
private void fastForwardAlarmToNext24Hours() {
// Get Alarm Time (unix)
DataStorageController alarmTimeController = new DataStorageController(getApplicationContext());
epochAlarmTime = alarmTimeController.getAlarmTime();
// Get Current Time (unix)
long epochCurrentTime = System.currentTimeMillis() / 1000L;
// While Alarm Time in the past, fast forward another 24 hours
while (epochAlarmTime < epochCurrentTime) {
epochAlarmTime = epochAlarmTime + (24*60*60); // Add a days worth of seconds
}
}
private Long getNumberOfSecondsUntilAlarm() {
long epochCurrentTime = System.currentTimeMillis() / 1000L;
return epochAlarmTime - epochCurrentTime;
}
This works fine and I get these sorts of results back:
SUMMER
10-12 07:46:26.678: D/CJS Logging(776): epochCurrentTime: 1381560386 10/12/2013 7:46:26 AM +1
10-12 07:46:26.678: D/CJS Logging(776): epochAlarmTime: 1381560480 10/12/2013 7:48:00 AM +1
However, when I forward the datetime of my device to the winter (post clocks change), I get the following result and the alarm fires 1 hour off:
WINTER
11-12 07:47:10.441: D/CJS Logging(942): epochCurrentTime: 1384242430 11/12/2013 7:47:10 AM +0
11-12 07:47:10.441: D/CJS Logging(942): epochAlarmTime: 1384325280 11/13/2013 6:48:00 AM +0
The reason this is happening is because the saved alarm time remains the same epoch time but when the clocks change that moves the local time by 1 hour.
Does anyone have any suggestions on how to deal with this?
Cheers, Charlie

Instead of using UNIX time, you may want to use local time instead, using Calendar instances:
// Get Current Time
Calendar currentTime = GregorianCalendar.getInstance();
// While Alarm Time in the past, fast forward a day
while (alarmTime.before(currentTime)) {
alarmTime.add(Calendar.DAY_OF_YEAR, 1);
}

Can you register for
http://developer.android.com/reference/android/content/Intent.html#ACTION_TIME_CHANGED
and update your next firing time?

The long value returned by System.currentTimeMillis() is always the the number of seconds after January 1, 1970 UTC.
Depending on how you initialize the timestamp, different values can displayed ( based on your locale/ machine / time etc ).
To keep it consistent, deal with your times internally using GMT ( or a timezone that does not have daylight savings ).

Difference in Timestamp in PHP and Java

I try to generate timestamp in PHP and Java(Android). Both needs to be same. But I am getting a difference of 3000+ between the time stamps. Please check the code below!
JAVA :
long unixTime = System.currentTimeMillis() / 1000L;
Php :
$time2 = time();

System.currentTimeMillis() will get me the current time in mobile and java.util.Date(); gets me based on GMT. Solved it.

Google Calendar API: Event endTime is decremented by 1 day

I'm trying to use the Google Calendar API in my own Java class. Unfortunately, the endTime of the newly created event (vacation in this case) seems to decremented by 1 day.
Example: I create an event with startTime 2011-01-01 and endTime 2011-01-05 the event will show up in Google Calendar from 2011-01-01 to 2011-01-04.
This is what I got so far (just the date part, taken from the Google Calendar API Developer's Guide, changed to Date because I want All Day events):
...
CalendarEventEntry myEntry = new CalendarEventEntry();
DateTime startTime = DateTime.parseDate("2011-01-01");
DateTime endTime = DateTime.parseDate("2011-01-05");
When eventTimes = new When();
eventTimes.setStartTime(startTime);
eventTimes.setEndTime(endTime);
myEntry.addTime(eventTimes);
Reminder reminder = new Reminder();
reminder.setMethod(Method.NONE);
myEntry.getReminder().add(reminder);
CalendarEventEntry insertedEntry = myService.insert(postUrl, myEntry);
...
Could this be somehow related to timezone issues? (I am from Germany)

When you don't provide DateTime.parseDate() with a time it will default to midnight. An event starting at midnight on the 1st and ending midnight on the 5th will display in the interface as running as full-day events from the 1st to the 4th. The time period doesn't include any time on the 5th, so it won't be displayed as being on the 5th.
You either need to set the end time as 2011-01-05 23:59, or add a day to the end date.