How to convert ZonedDateTime to milliSecond in Java?

I'm trying to convert ZonedDateTime to milliseconds using below code.

LocalDateTime ldt = LocalDateTime.now();
ZonedDateTime zonedDateTime =ldt.atZone(ZoneId.of(""Asia/Kolkata""));
zonedDateTime.toInstant().toEpochMilli();

But this one returns milliseconds based on local time. And it's not considering ZoneId.

Let's say LocalDateTime("2019-04-10T05:30"). If I convert this to ZonedDateTime with Zone id ("Asia/Kolkata") then I'm getting ("2019-04-10T05:30+05:30[Asia/Kolkata]"). Then I convert to EpochMilli (1554854400000) = ("Wed Apr 10 2019 00:00:00") in UTC.

Solution

You are using an Instant to get that milliseconds representation. Instant are not zone based. Now, the epoch time is based on the "1970-01-01T00:00:00Z" so you should not have the zone in it.

If you want to create a ZoneDateTime from the epoch value, you can simply create an Instant at that epoch time and then create a ZonedDateTime with the zone you wish :

//Let's create our zone time (just to keep your logic
LocalDateTime ldt = LocalDateTime.now();
ZonedDateTime zonedDateTime =ldt.atZone(ZoneId.of("Asia/Kolkata"));

//Then get the epoch on GMT
long e = zonedDateTime.toInstant().toEpochMilli();

Instant i = Instant.ofEpochMilli(e);
System.out.println(ZonedDateTime.ofInstant(i, ZoneId.systemDefault()));
System.out.println(ZonedDateTime.ofInstant(i, ZoneId.of("Asia/Kolkata")));

2019-04-12T05:10:31.016+02:00[Europe/Paris]
2019-04-12T08:40:31.016+05:30[Asia/Kolkata]

NOTE : The code above should not be used like this, it is not necessary to get a LocalDateTime then a ZonedDateTime to finally create an Instant. This is just to show that even with a zone, this will be "lost" at one point.
Simply use :

long e = Instant.now().toEpochMilli();