Format ZonedDateTime and return ZonedDateTime

Question

I have below ZonedDateTime object:

ZonedDateTime z1 = ZonedDateTime.parse("2021-08-06T19:01:32.632+05:30[Asia/Calcutta]");

Need to return another ZonedDateTime object with value: 06-Aug-2021 19:01:32 IST

I can format it like below:

DateTimeFormatter df = DateTimeFormatter.ofPattern("dd-MMM-yyyy HH:mm:ss zz");
 String s = z1.withZoneSameInstant(ZonedDateTime.now().getZone()).format(df);

but I need to return ZonedDateTime not String. How do I now convert it back to ZonedDateTime. If I parse again timezone shifts or time value is changed.

Answer 1

ZonedDateTime holds no format, no text

but I need to return ZonedDateTime not String.

Short answer: It is not possible.

Explanation:

The standard Date-Time classes do not have any attribute to hold the formatting information. Even if some library or custom class promises to do so, it is breaking the Single Responsibility Principle. A Date-Time object is supposed to store the information about Date, Time, Timezone etc., not about the formatting. The only way to represent a Date-Time object in the desired format is by formatting it into a String using a Date-Time parsing/formatting type:

• For the modern Date-Time API: java.time.format.DateTimeFormatter
• For the legacy Date-Time API: java.text.SimpleDateFormat

The 3-letter abbreviation for the timezone ID:

The problem with the 3-letter abbreviation e.g. IST is that it can specify many timezones; therefore, you need to change it to the desired timezone ID as per the naming convention, Region/City. Given below is an excerpt from the documentation:

Three-letter time zone IDs

For compatibility with JDK 1.1.x, some other three-letter time zone IDs (such as "PST", "CTT", "AST") are also supported. However, their use is deprecated because the same abbreviation is often used for multiple time zones (for example, "CST" could be U.S. "Central Standard Time" and "China Standard Time"), and the Java platform can then only recognize one of them.

Demo:

import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        ZonedDateTime z1 = ZonedDateTime.parse("2021-08-06T19:01:32.632+05:30[Asia/Calcutta]");
        DateTimeFormatter df = DateTimeFormatter.ofPattern("dd-MMM-yyyy HH:mm:ss z", Locale.ENGLISH);
        String s = z1.format(df);
        System.out.println(s);

        // Getting the original ZonedDateTime from the string
        s = s.replace("IST", "Asia/Calcutta");
        ZonedDateTime z2 = ZonedDateTime.parse(s, df);
        System.out.println(z2);
    }
}

Output:

06-Aug-2021 19:01:32 IST
2021-08-06T19:01:32+05:30[Asia/Calcutta]

ONLINE DEMO

Answer 2

Why not to just use overloaded version parse(CharSequence t, DateTimeFormatter f), as:

ZonedDateTime z1 = ZonedDateTime
        .parse("2021-08-06T19:01:32.632+05:30[Asia/Calcutta]", DateTimeFormatter.ISO_ZONED_DATE_TIME);

Or, if you instist you want to have two distinct objects:

String zdt = "2021-08-06T19:01:32.632+05:30[Asia/Calcutta]";

ZonedDateTime z1 = ZonedDateTime.parse(zdt);
ZonedDateTime z2 = ZonedDateTime.parse(zdt, DateTimeFormatter.ISO_ZONED_DATE_TIME);