Q:

Unable to convert a given Local Date to GMT Date using Java 8

I'm having hard time converting a given local date (which is in IST) to GMT using Java 8 classes LocalDateTime and ZonedDateTime.
Consider the following code snippet.
LocalDateTime ldt = LocalDateTime.parse("22-1-2015 10:15:55 AM", DateTimeFormatter.ofPattern("dd-M-yyyy hh:mm:ss a"));

ZonedDateTime gmtZonedTime = ldt.atZone(ZoneId.of("GMT+00"));

System.out.println("Date (Local) : " + ldt);

System.out.println("Date (GMT) : " + gmtZonedTime);

Which produces the following output:
Date (Local) : 2015-01-22T10:15:55
Date (GMT) : 2015-01-22T10:15:55

As I perceive this, it is only converting the format, not the time. 
The output I expect is this: (As GMT is 5.30 hours behind IST, for instance)
Date (Local) : 2018-02-26T01:30
Date (GMT) : 2018-02-26T08:00Z[GMT]

Please guide me get there!

A:

LocalDateTime ldt = LocalDateTime.parse("22-1-2015 10:15:55 AM",
            DateTimeFormatter.ofPattern("dd-M-yyyy hh:mm:ss a"));

Instant instant = ldt.atZone(ZoneId.of("GMT+05:30")).toInstant(); // get instant in timeLine by mentioning the zoneId at which you have obtained the ldt

System.out.println("Date (Local) : " + ldt);

System.out.println("Date (GMT) : " + LocalDateTime.ofInstant(instant, ZoneId.of("GMT")));

A:

The output is what you should expect. A LocalDateTime does not have a time zone at all, it just has the date and time somewhere (not specified). Your assumption that it is in IST is wrong, it is not. The atZone method adds a time zone, in this case UTC/GMT, but it does not change the date or time.
See https://docs.oracle.com/javase/8/docs/api/java/time/LocalDateTime.html.
You probably want to convert the LocalDateTime to a ZonedDateTime for IST first and then change the time zone to UTC. That should change the time.

