IST mapped to wrong ZoneId in java.time library

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First, if there’s any way you can avoid it, don’t rely on three letter time zone abbreviations. They are ambiguous, probably more often than not. When you say that IST should be mapped to Asia, it still leaves the choice between Asia/Tel_Aviv and Asia/Kolkata open (plus the aliases for those two, Asia/Jerusalem and Asia/Calcutta). In other places in the world IST may mean Irish Summer Time and apparently also Iceland Standard Time (or something like it; it certainly makes sense). It’s not the first time I have seen IST recognized as Atlantic/Reykjavik.

If you can’t avoid having to parse IST, control the interpretation through the two-arg appendZoneText method of a DateTimeFormatterBuilder. It accepts a set of preferred zones:

DateTimeFormatter DATE_WITH_TIMEZONE_FORMATTER = new DateTimeFormatterBuilder()
        .appendPattern("yyyy-MM-dd HH:mm ")
        .appendZoneText(TextStyle.SHORT, Collections.singleton(ZoneId.of("Asia/Kolkata")))
        .toFormatter(Locale.ENGLISH);

Substitute your preferred preferred zone where I put Asia/Kolkata. The rest shouldn’t cause trouble:

    String timeInput = "2019-08-29 00:00 IST";
    ZonedDateTime zdt = ZonedDateTime.parse(timeInput, DATE_WITH_TIMEZONE_FORMATTER);
    System.out.println("Parsed ZonedDateTime: "+ zdt);
    long timestamp = zdt
            .toInstant().toEpochMilli();
    System.out.println("timestamp: " + timestamp);

Output:

Parsed ZonedDateTime: 2019-08-29T00:00+05:30[Asia/Kolkata]
timestamp: 1567017000000

Link: Time Zone Abbreviations – Worldwide List (you will notice that IST comes three times in the list and that many other abbreviations are ambiguous too).

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