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Java: Fix incorrect timezone in date object

An external API returns an object with a date.
According to their API specification, all dates are always reported in GMT.
However, the generated client classes (which I can't edit) doesn't set the timezone correctly. Instead, it uses the local timezone without converting the date to that timezone.
So, long story short, I have an object with a date that I know to be GMT but it says CET. How can I adjust for this mistake withouth changing my local timezone on the computer or doing something like this:
LocalDateTime.ofInstant(someObject.getDate().toInstant().plus(1, ChronoUnit.HOURS),
ZoneId.of("CET"));
Thank you.

tl;dr ⇒ use ZonedDateTime for conversion
public static void main(String[] args) {
// use your date here, this is just "now"
Date date = new Date();
// parse it to an object that is aware of the (currently wrong) time zone
ZonedDateTime wrongZoneZdt = ZonedDateTime.ofInstant(date.toInstant(), ZoneId.of("CET"));
// print it to see the result
System.out.println(wrongZoneZdt.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
// extract the information that should stay (only date and time, NOT zone or offset)
LocalDateTime ldt = wrongZoneZdt.toLocalDateTime();
// print it, too
System.out.println(ldt.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
// then take the object without zone information and simply add a zone
ZonedDateTime correctZoneZdt = ldt.atZone(ZoneId.of("GMT"));
// print the result
System.out.println(correctZoneZdt.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
}
Output:
2020-01-24T09:21:37.167+01:00[CET]
2020-01-24T09:21:37.167
2020-01-24T09:21:37.167Z[GMT]
Explanation:
The reason why your approach did not just correct the zone but also adjusted the time accordingly (which is good when desired) is your use of a LocalDateTime created from an Instant. An Instant represents a moment in time which could have different representations in different zones but it stays the same moment. If you create a LocalDateTime from it and put another zone, the date and time are getting converted to the target zone's. This is not just replacing the zone while keeping the date and time as they are.
If you use a LocalDateTime from a ZonedDateTime, you extract the date and time representation ignoring the zone, which enables you to add a different zone afterwards and keep the date and time as it was.
Edit: If the code is running in the same JVM as the faulty code, you can use ZoneId.systemDefault() to get the same time zone as the faulty code is using. And depending on taste you may use ZoneOffset.UTC instead of ZoneId.of("GMT").

I am afraid you will not get around some calculations here. I'd strongly suggest to follow an approach based on java.time classes, but alternatively you might use the java.util.Calendar class and myCalendar.get(Calendar.ZONE_OFFSET) for those calculations:
https://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html#ZONE_OFFSET

How to find epoch format current time of GMT using java

I have written below code which is running, and giving output. But I'm not sure It's a right one.
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
Date date = new Date();
sdf.setTimeZone(TimeZone.getTimeZone("GMT-7"));
String value = sdf.format(date);
System.out.println(value);
Date date2 = sdf.parse(value);
long result = date2.getTime();
System.out.println(result);
return result;
The above code what I'm trying is, I just need to get the current time of GMT time zone and convert it as epoch format which is gonna used in Oracle db.
Can someone tell me that format, and the above code is right?

First, you should not store time since the epoch as a timestamp in your database. Look into the date-time datatypes that your DMBS offers. In Oracle I think that a date column will be OK. For most other DBMS you would need a datetime column. timestamp and timestamp with timezone may be other and possibly even sounder options depending on your exact requirements.
However, taking your word for it: Getting the number of milliseconds since the epoch is simple when you know how:
long millisecondsSinceEpoch = System.currentTimeMillis();
System.out.println(millisecondsSinceEpoch);
This just printed:
1533458641714
The epoch is defined in UTC, so in this case we need to concern ourselves with no other time zones.
If you needed seconds rather than milliseconds, it’s tempting to divide by 1000. However, doing your own time conversions is a bad habit since the libraries already offers them, and using the appropriate library methods gives clearer, more explanatory and less error-prone code:
long secondsSinceEpoch = Instant.now().getEpochSecond();
System.out.println(secondsSinceEpoch);
1533458641
You said:
I just need to get the current time of GMT time zone…
Again taking your word:
OffsetDateTime currentTimeInUtc = OffsetDateTime.now(ZoneOffset.UTC);
System.out.println(currentTimeInUtc);
long millisecondsSinceEpoch = currentTimeInUtc.toInstant().toEpochMilli();
System.out.println(millisecondsSinceEpoch);
2018-08-05T08:44:01.719265Z
1533458641719
I know that GMT and UTC are not exactly the same, but for most applications they can be (and are) used interchangeably.
Can someone tell me (if) the above code is right?
When I ran your code just now, its output agreed with mine except the milliseconds were rounded down to whole thousands (whole seconds):
1533458641000
Your code has some issues, though:
You are using the old, long out-dated and poorly designed classes SimpleDateFormat, Date and TimeZone. The first in particular has a reputation for being troublesome. Instead we should use java.time, the modern Java date and time API.
Bug: In your format pattern string you are using lowercase hh for hour of day. hh is for hour within AM or PM, from 1 through 12, so will give you incorrect results at least half of the day. Uppercase HH is for hour of day.
Don’t use GMT-7 as a time zone. Use for example America/Los_Angeles. Of course select the time zone that makes sense for your situation. Edit: You said:
I just want to specify the timezone for sanjose. GMT-7 is refer to
sanjose current time.
I believe many places are called San Jose. If you mean San Jose, California, USA, you are going to modify your program to use GMT-8 every time California goes back to standard time and opposite when summer time (DST) begins?? Miserable idea. Use America/Los_Angeles and your program will work all year.
Since you ask for time in the GMT time zone, what are you using GMT-7 for at all?
There is no point that I can see in formatting your Date into a string and parsing it back. Even if you did it correctly, the only result you would get would be to lose your milliseconds since there are no milliseconds in your format (it only has second precision; this also explained the rounding down I observed).
Links
Oracle tutorial: Date Time explaining how to use java.time, the modern Java date and time API.
San Jose, California on Wikipedia

Why not use Calendar class?
public long getEpochTime(){
return Calendar.getInstance(TimeZone.getTimeZone("GMT-7")).getTime().getTime()/1000; //( milliseconds to seconds)
}
It'll return the current Date's Epoch/Unix Timestamp.
Based on Harald's Comment:
public static long getEpochTime(){
return Clock.system(TimeZone.getTimeZone("GMT-7").toZoneId() ).millis()/1000;
}

Here is a solution using the java.time API
ZonedDateTime zdt = LocalDateTime.now().atZone(ZoneId.of("GMT-7"));
long millis = zdt.toInstant().toEpochMilli();

What is the right way to format Time between different timezones?

I want to format time like 19:19:00 to different time zones. If I use SimpleDateFormat it always takes into account the start of the epoch: 1970.01.01.
Some timezones have different offsets on the start of the epoch and now. For example, the default offset from Europe/Kiev now is UTC+0200 but in 1970 it was UTC+0300. That means if I run my server under Europe/Kiev the client which login under Europe/Berlin(UTC+0100) will see three hours different instead of two.
I can solve this problem by writing a custom formatter for java.sql.Time. But I want to ask maybe there are some common approach or Java tools/libraries which can solve it.
Another solution can be using joda-time:
TimeZone.setDefault(TimeZone.getTimeZone("Europe/Kiev"));
DateTimeZone.setDefault(DateTimeZone.forID("Europe/Kiev"));
DateTimeFormat.forPattern("HH:mm:ss.SSS")
.withZone(DateTimeZone.forID("Europe/Berlin"))
.print(Time.valueOf("19:00:00").getTime());

You can't format just a time to different time zones. You need a date.
If you want to assume that the date of that time is today, you can try this code:
ZoneId originalZone = ZoneId.of("Europe/Kiev");
ZoneId targetZone = ZoneId.of("Europe/Berlin");
LocalTime originalTime = LocalTime.parse("19:19:00");
LocalTime convertedTime = LocalDate.now(originalZone)
.atTime(originalTime)
.atZone(originalZone)
.withZoneSameInstant(targetZone)
.toLocalTime();
System.out.println(convertedTime);

Is java.time.instant an alternative for you? It handles all Timestamps internally as UTC-Time.
One way to parse it from a string is Instant.parse("2018-05-30T19:00:00")
If you want to have the time for a specific timezone you can get it with myInstant.atZone("Zone")

ZoneId originalZone = ZoneId.of("Europe/Kiev");
ZoneId targetZone = ZoneId.of("Europe/Berlin");
LocalDate assumedDate = LocalDate.now(originalZone);
String formattedTime = assumedDate.atTime(LocalTime.parse("19:19:00"))
.atZone(originalZone)
.withZoneSameInstant(targetZone)
.format(DateTimeFormatter.ofPattern("HH:mm:ss"));
System.out.println(formattedTime);
Today this printed:
18:19:00
When you know the date, you should of course use that instead of just today. In the case of Kyiv and Berlin I think they follow the same rules for summer time (DST), so the precise date may not be important. If converting between zones that don’t use the same transitions, or between a time zone that uses summer time and one that doesn’t, it’s suddenly crucial. And who knows in which of those two countries the politicians will change the rules next year? Better be safe.
Link: Oracle tutorial: Date Time explaining how to use java.time.

Convert database time to IST datetime

I have a time value stored in my database in HH:mm:ss format (using MySQL's time type). This time is to be considered as a value of IST timezone. The server on which my Java code runs follows the UTC timezone.
How can I get a formatted datetime in yyyy-MM-dd HH:mm:ss in IST (or in UTC millis)? Following is what I've tried till now:
// ... Code truncated for brevity
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalTime time = resultSet.getTime("send_time").toLocalTime();
LocalDateTime datetime = time.atDate(LocalDate.now());
System.out.println(datetime.format(formatter));
The above correctly prints the datetime on my local machine, which is on IST, but I'm concerned about how it will behave on the remote server.

Your approach is fine and should work regardless of your computer's time zone since there is no time zone information in either LocalTime or LocalDateTime. One possible issue is with LocalDate.now() which returns today's date in the computer's local time zone, not in IST. You may want to replace it with LocalDate.now(ZoneId.of("Asia/Calcutta")).
Or as commented by #OleV.V. you could use the new driver facilities to derive a LocalTime directly:
LocalTime time = resultSet.getObject("send_time", LocalTime.class);
Note possible caveats with your approach:
if the time zone you use introduces DST, you may end up with two identical times in your DB that were actually different instants - using UTC to store times is probably more robust
time in mysql can store values smaller than 00:00 and larger than 23:59:59.999999, in which case you may experience unexpected behaviours on the Java side.

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