How to convert Sting into instant in Java

Question

How to convert this string into instant: String date = "Fri Sep 30 00:00:00 IST 2022";

Exception:

java.time.format.DateTimeParseException: Text 'Fri Sep 30 00:00:00 IST 2022' could not be parsed at index 0
    at java.base/java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:2046)
    at java.base/java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1948)
    at java.base/java.time.LocalDate.parse(LocalDate.java:428)

In DB it is saved in this format

ISODate("2022-09-29T18:30:00.000Z")

But while debugging in IDE it is coming in string format like this: "Fri Sep 30 00:00:00 IST 2022" Now I want to convert it back to instant

I tried in this way:

DateTimeFormatter DATE_FORMAT_RULE = DateTimeFormatter.ofPattern("MM/dd/yyyy");
String date = "Fri Sep 30 00:00:00 IST 2022";
Instant instant = LocalDate.parse(date, DATE_FORMAT_RULE)
                           .atStartOfDay()
                           .toInstant(ZoneOffset.UTC);

Answer 1

Works for me

        DateTimeFormatter DATE_FORMAT_RULE = DateTimeFormatter.ofPattern("E MMM dd hh:mm:ss z yyyy", Locale.US);
        String date = "Fri Sep 30 00:00:00 IST 2022";
        Instant instant2 = LocalDate.parse(date, DATE_FORMAT_RULE)
                .atStartOfDay()
                .toInstant(ZoneOffset.UTC);

Answer 2

The pattern you are using does not match the one of the String. Your pattern cannot parse a two-digit month first because the String starts with an abbreviated day of week, whose abbreviation even depends on the Locale, which you haven't specified, so it will take the system default.

However, you can try to parse it with a different pattern, but I don't think it will get you the desired result… Try DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss z uuuu", Locale.ENGLISH) instead of your DateTimeFormatter.ofPattern("MM/dd/yyyy") and see the result. On my machine, it assumes IST to be Iceland Standard Time!

See this example:

public static void main(String[] args) {
    // example String
    String toBeParsed = "Fri Sep 30 00:00:00 IST 2022";
    // first formatter: tries to parse "IST" as ZONE TEXT
    DateTimeFormatter dtfZ = new DateTimeFormatterBuilder()
                                    .appendPattern("EEE MMM dd HH:mm:ss")
                                    .appendLiteral(' ')
                                    .appendZoneText(TextStyle.SHORT)
                                    .appendLiteral(' ')
                                    .appendPattern("uuuu")
                                    .toFormatter(Locale.ENGLISH);
    // second formatter: tries to parse "IST" as ZONE NAME
    DateTimeFormatter dtfz = DateTimeFormatter.ofPattern(
                                    "EEE MMM dd HH:mm:ss z uuuu",
                                    Locale.ENGLISH
                            );
    // parse to a ZonedDateTime with the first formatter
    ZonedDateTime zdt = ZonedDateTime.parse(toBeParsed, dtfZ);
    // print the result
    System.out.println("ZonedDateTime: "
                    + zdt.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
    // parse to a ZonedDateTime with the second formatter
    ZonedDateTime zdt2 = ZonedDateTime.parse(toBeParsed, dtfz);
    // print that, too
    System.out.println("ZonedDateTime: "
                    + zdt2.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
    // convert to an Instant
    Instant instant = zdt.toInstant();
    // print the epoch millis
    System.out.println(instant.toEpochMilli());
}

Output:

ZonedDateTime: 2022-09-30T00:00:00Z[Atlantic/Reykjavik]
ZonedDateTime: 2022-09-30T00:00:00Z[Atlantic/Reykjavik]
1664496000000

If you want the LocalDate.atStartOfDay(), you could simply extract it by calling toLocalDate() on an instance of ZonedDateTime (in case that ZonedDateTime contains any time of day different from zero hours, minutes, seconds and further down the units).

Answer 3

Corresponding to 2022-09-29T18:30:00.000Z, the IST in Fri Sep 30 00:00:00 IST 2022 refers to Indian Standard Time which has a time offset of UTC+05:30. You can build a formatter using .appendZoneText(TextStyle.SHORT, Set.of(ZoneId.of("Asia/Kolkata"))) with DateTimeFormatterBuilder as shown in the demo below:

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter formatter = new DateTimeFormatterBuilder()
                .appendPattern("EEE MMM dd HH:mm:ss")
                .appendLiteral(' ')
                .appendZoneText(TextStyle.SHORT, Set.of(ZoneId.of("Asia/Kolkata")))
                .appendLiteral(' ')
                .appendPattern("uuuu")
                .toFormatter(Locale.ENGLISH);

        Instant instant = ZonedDateTime.parse("Fri Sep 30 00:00:00 IST 2022", formatter).toInstant();
        System.out.println(instant);
    }
}

Output:

2022-09-29T18:30:00Z

Learn more about the the modern date-time API from Trail: Date Time.