Convert UTC time to local time while using the local year/month/day and specifying the hours/minutes JAVA

Question

I'm trying to convert 09:00 UTC into the local equivalent like this:

    ZoneId localZoneId = ZoneId.of(TimeZone.getDefault().getID());
    DateTimeFormatter formatt = DateTimeFormatter.ofPattern("HH:mm").withZone(localZoneId);
    ZonedDateTime test = ZonedDateTime.of( 2020 , 4 , 25 , 9 , 0 , 0 , 0 , ZoneId.of("UTC"));
        String test2 = formatt.format(test);
        System.out.println(test);
        System.out.println(test2);

Output

2020-04-25T09:00Z[UTC]
02:00

However, instead of manually entering the the year, month and day I want to grab the current year, month, day from the local machine but still keep the hour/minutes at 09:00.this part of the code needs to work like this

    ZonedDateTime test = ZonedDateTime.of( currentYear, currentMonth , currentDay, 9 , 0 , 0 , 0 , ZoneId.of("UTC"));

Was thinking of doing this but it seems like alot of code:

        ZonedDateTime dateparse= ZonedDateTime.now();
        ZoneId localZoneId = ZoneId.of(TimeZone.getDefault().getID());
        DateTimeFormatter formatFullLocal = DateTimeFormatter.ofPattern("HH:mm").withZone(localZoneId);


        DateTimeFormatter year = DateTimeFormatter.ofPattern("yy");
        String localMachineYearString= year.format(dateparse);
        int localMachineYearInt = Integer.parseInt(localMachineYearString);


        DateTimeFormatter month= DateTimeFormatter.ofPattern("M");
        String localMachineMonthString= month.format(dateparse);
        int localMachineMonthInt= Integer.parseInt(localMachineMonthString);


        DateTimeFormatter day= DateTimeFormatter.ofPattern("dd");
        String localMachineDayString= day.format(dateparse);
        int localMachineDayInt= Integer.parseInt(localMachineDayString);


        ZonedDateTime test =ZonedDateTime
                .of(localMachineYearInt, localMachineMonthInt , localMachineDayInt , 9 , 0 , 0 , 0 , ZoneId.of("UTC"));

Thank You!

Answer 1

tl;dr

ZonedDateTime.now().with( LocalTime.of( 9 , 0 ) )

The LocalTime object, when passed to the with method, acts as a TemporalAdjuster , to move to a different date-time value. That value is delivered in a fresh new object rather than altering the original, as immutable objects .

The line of code above depends implicitly on the JVM's current default time zone. Better to specify explicitly.

Details

By the way, do not mix legacy date-time classes with java.time classes. Avoid the legacy classes entirely. So this:

ZoneId localZoneId = ZoneId.of(TimeZone.getDefault().getID());

…should be:

ZoneId localZoneId = ZoneId.systemDefault() ;

Also, “local” in java.time means “not zoned”, or the zone/offset is unknown or unspecified. So your variable name localZoneId is confusing. Should be something like this:

ZoneId zoneId = ZoneId.systemDefault() ;

---

You said:

I want to grab the current year, month, day from the local machine

Determining the current date requires a time zone. For any given moment, the date varies around the globe by time zone.

ZoneId z = ZoneId.systemDefault() ;
LocalDate today = LocalDate.now( z ) ;

You said:

but still keep the hour/minutes at 09:00

A ZonedDateTime represents a date and a time-of-day, in the context of a time zone. You can specify each of those three parts in the factory method ZonedDateTime.of .

LocalTime lt = LocalTime.of( 9 , 0 ) ;
ZonedDateTime zdt = ZonedDateTime.of( today , lt , z ) ;  // Pass date, time, zone.