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Local timezone date object to UTC timezone date object [duplicate]

This question already has answers here:
Calendar returns date in wrong time zone
(5 answers)
Closed 3 years ago.
I am trying to convert my datetime that is in local timezone into UTC date time.
Date localDate; // this is local date
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZZZZZ") ;
simpleDateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
String dateStr = simpleDateFormat.format(localDate);
i am getting proper converted UTC time in dateStr now i want to convert it into Date object with UTC timezone only
but the moment i do that i am again getting the localDate.
//converting string to date object
simpleDateFormat.parse(dateStr)
does anyone know how can i convert local date object to UTC date object
here is the value i am getting while debugging
here dateStr is showing proper date in UTC but utcDate object is showing the local time

Date class is intended to reflect coordinated universal time (UTC) time. It can be formatted into ANY form you want, e.g. you can format it into your local time zone or UTC time zone.
See from javadoc: https://docs.oracle.com/javase/8/docs/api/java/util/Date.html
Although the Date class is intended to reflect coordinated universal time (UTC), it may not do so exactly, depending on the host environment of the Java Virtual Machine. Nearly all modern operating systems assume that 1 day = 24 × 60 × 60 = 86400 seconds in all cases. In UTC, however, about once every year or two there is an extra second, called a "leap second." The leap second is always added as the last second of the day, and always on December 31 or June 30. For example, the last minute of the year 1995 was 61 seconds long, thanks to an added leap second. Most computer clocks are not accurate enough to be able to reflect the leap-second distinction.

Jboss Java Date daylight saving time

Have an issue where, when clocks are moved due to a Daylight savings time (twice a year), dates are not correct in Java (I am based in Central Europe: GMT+2 in summer, GMT+1 in winter)
If time is moved 1 hour ahead, new Date() still returns old time (1 hour behind of current time).
In Java 7, can this be solved, without restarting the Jboss application servers?
If I change the time manually in Windows, reproduce the problem: Date is not updated to the system date unless jboss is restarted.
Calendar c = Calendar.getInstance();
c.setTime(new Date());

In Java <= 7, you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes.
With this, you can handle DST changes easily.
First, you can use the org.threeten.bp.DateTimeUtils to convert from and to Calendar.
The following code converts the Calendar to org.threeten.bp.Instant, which is a class that represents an "UTC instant" (a timestamp independent of timezone: right now, at this moment, everybody in the world are in the same instant, although their local date and time might be different, depending on where they are).
Then, the Instant is converted to a org.threeten.bp.ZonedDateTime (which means: at this instant, what is the date and time at this timezone?). I also used the org.threeten.bp.ZoneId to get the timezone:
Calendar c = Calendar.getInstance();
c.setTime(new Date());
// get the current instant in UTC timestamp
Instant now = DateTimeUtils.toInstant(c);
// convert to some timezone
ZonedDateTime z = now.atZone(ZoneId.of("Europe/Berlin"));
// today is 08/06/2017, so Berlin is in DST (GMT+2)
System.out.println(z); // 2017-06-08T14:11:58.608+02:00[Europe/Berlin]
// testing with a date in January (not in DST, GMT+1)
System.out.println(z.withMonth(1)); // 2017-01-08T14:11:58.608+01:00[Europe/Berlin]
I've just picked some timezone that uses Central Europe timezone (Europe/Berlin): you can't use those 3-letter abbreviations, because they are ambiguous and not standard. You can change the code to the timezone that suits best for your system (you can get a list of all available timezones with ZoneId.getAvailableZoneIds()).
I prefer this solution because it's explicit what timezone we're using to display to the user (Date and Calendar's toString() methods use the default timezone behind the scenes and you never know what they're doing).
And internally, we can keep using the Instant, which is in UTC, so it's not affected by timezones (and you can always convert to and from timezones whenever you need) - if you want to convert the ZonedDateTime back to an Instant, just use the toInstant() method.
Actually, if you want to get the current date/time, just forget the old classes (Date and Calendar) and use just the Instant:
// get the current instant in UTC timestamp
Instant now = Instant.now();
But if you still need to use the old classes, just use DateTimeUtils to do the conversions.
The output of the examples above are the result of the ZonedDateTime.toString() method. If you want to change the format, use the org.threeten.bp.format.DateTimeFormatter class (take a look at the javadoc for more details about all the possible formats):
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy HH:mm:ss z X");
// DST (GMT+02)
System.out.println(formatter.format(z)); // 08/06/2017 14:11:58 CEST +02
// not DST (GMT+01)
System.out.println(formatter.format(z.withMonth(1))); // 08/01/2017 14:11:58 CET +01

Use ZonedDateTime class from JDK 8 java.time. It accommodates the Daylight Saving Time changes.
Refer the details at : https://docs.oracle.com/javase/8/docs/api/java/time/ZonedDateTime.html

Ignoring DST when using Java Calendars

I have a GUI that plots time-series graphs. The user enters the dates they want to plot from and to by entering values in text boxes. For example, if they enter 25/07/13 22:00 and 26/07/13 00:00 the graph should plot data from 22:00:00 on the 25th through to 00:00:59 the following morning. The times the user enters are parsed into a Calendar object.
My problem comes with DST.
The user doesn't care about DST, so all they want to see is a graph between those two times. However, the Calendar objects do care about DST and so my "to" date is currently not 00:00, but 01:00. (I am in London and we are currently GMT + 1 hour for DST).
I want to effectively ignore DST and act as though everything is GMT + 0 when I want to plot graphs.
How can I do this?

Thanks for the answers guys, they helped me get my head around the problem. It sort of comes down to the fact that I use both the Calendar object, for presentation and storage of data, and the epoch for temporal calculations.
It turns out that the Calendar set() methods will take into account DST as a matter of course. So when I parse the time values in the text boxes that the user enters, and use set() for each individual Calendar field, the Calendar object will know, based-off historical data, whether the date you've just set will have DST applied. Because of this, it understands that you meant, for example, GMT+1, even if you didn't realise it (because, frankly, who does?!).
However, when you do getTimeInMillis(), the epoch returned has no concept of time zones or DST, so to match with the current time zone you have to apply DST manually to the returned epoch, if it applies. Conversely, when you use setTimeInMillis() on a Calendar object, it is assumed that the time you entered is GMT+0, but if the epoch is a date that currently has DST applied, the Calendar object will add it on for you, meaning you're +1 hour from where you thought you were. To solve this problem, you need to subtract DST, again if necessary, from the epoch before setting it in the calendar.
All of this confusion is particularly important on day boundaries, especially if you're using day resolution for anything, like me.

Calendar calendar = Calendar.getInstance();
calendar.set(calendar.DST_OFFSET, 0);

If I understand you correctly you need to parse 25/07/13 22:00 as GMT date/time:
SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yy HH:mm");
sdf.setTimeZone(TimeZone.getTimeZone("GMT"));
Date date = sdf.parse("25/07/13 22:00");
and make a Calendar based on this date
Calendar c= Calendar.getInstance();
c.setTime(date);

TimeZone tz = TimeZone.getTimeZone("Etc/GMT0");
DateFormat df = DateFormat.getDateTimeInstance();
df.setTimeZone(tz);
System.out.println(df.format(new Date()));

Is java.util.Date represents date as system date or not? [duplicate]

This question already has answers here:
Closed 10 years ago.
I need to get real world date and time in Java. I use:
Date date = new Date();
But I'm not sure that it is not just system time. I don't need to be dependent on PC local date and time.
If it is so, then is there any way to abstract from it? I mean I need correct time and date. If today is the 1st of May, 2012 and user changed (maybe there was a system error) it to the 1st of December 2000, it shouldn't affect business logic. So is there any alternative to achieve this?

Date only represents an instant in time, in milliseconds since the Unix epoch of January 1st 1970 UTC (modulo leap seconds). It has no concept of a time zone in its data. However, if you use the toString method it will always convert that UTC instant to a local date/time using the system time zone. That confuses a lot of users, making them think that Date contains a time zone - it's just an illusion.
Likewise Date doesn't have any concept of a calendar system (Gregorian, Julian etc) or a "format". Basically it's just a long :)

How can I print a Date without timezone?

I have a mobile application where I capture a date/time and send it as miliseconds to a servlet. -For example I capture 12:55 AM in my cellphone and send it to my server-
So, I construct the date from the sent miliseconds and print it with SimpleDateFormat but the time is diferent, it prints 8:55 AM.
I know that the problem is because I use 2 diferent timezones, my question is:
how can I show that time without apply any timezone to show the same time?
Thanks in advance

You need to use Calendar to change the TimeZone but there is no API for that.
// Change a date in another timezone
public static Date changeTimeZone(Date date, TimeZone zone) {
Calendar first = Calendar.getInstance(zone);
first.setTimeInMillis(date.getTime());
Calendar output = Calendar.getInstance();
output.set(Calendar.YEAR, first.get(Calendar.YEAR));
output.set(Calendar.MONTH, first.get(Calendar.MONTH));
output.set(Calendar.DAY_OF_MONTH, first.get(Calendar.DAY_OF_MONTH));
output.set(Calendar.HOUR_OF_DAY, first.get(Calendar.HOUR_OF_DAY));
output.set(Calendar.MINUTE, first.get(Calendar.MINUTE));
output.set(Calendar.SECOND, first.get(Calendar.SECOND));
output.set(Calendar.MILLISECOND, first.get(Calendar.MILLISECOND));
return output.getTime();
}
Link: http://blog.vinodsingh.com/2009/03/date-and-timezone-in-java.html
I think this should work. I haven't extensively tested it.

Calendar and Date objects store their date information in milliseconds in relation to UTC.
(the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC)
This means that Calendar/Date Objects do not store different values for different Time Zones. They always use the same value internally... the format is what normally changes.
Maybe you can use a SimpleDateFormat in your local/default timezone:
SimpleDateFormat sdf = new SimpleDateFormat("S")
Date d = sdf.parse(milliseconds);
You can also try to change the DateFormat's timezone until it matches your expected output.
sdf.setTimeZone(TimeZone.getTimeZone("GMT-8"));
System.out.println(sdf.toString());
sdf.setTimeZone(TimeZone.getTimeZone("GMT-4"));
System.out.println(sdf.toString());