How to round it down to the closest 5 minute interval if it is 1,2,3,4 minutes ahead?

I want to round down an Instant / LocalDateTime to its closest 5 minutes interval in Java.

Examples: Suppose the time is:

2021-02-08T19:01:49.594 

or

2021-02-08T19:02:49.594

or

2021-02-08T19:03:49.594

or

2021-02-08T19:04:49.594

Expected result:

2021-02-08T19:00:00.000

You can truncate it to ChronoUnit.MINUTES and then check the minute-of-hour as per the requirement i.e. if it is not a multiple of 5 subtract the remainder when divided by 5. Use LocalDate#withMinute to return a copy of this LocalDateTime with the minute-of-hour altered.

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;

class Main {
    public static void main(String[] args) {
        // Test
        String[] arr = { "2021-02-08T19:02:49.594", "2021-02-08T19:56:49.594", "2021-02-08T19:54:49.594",
                "2021-02-08T19:06:49.594" };

        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ss.SSS");

        for (String s : arr) {
            System.out.println(roundToNearestHour(s).format(dtf));
        }
    }

    static LocalDateTime roundToNearestHour(String str) {
        LocalDateTime ldt = LocalDateTime.parse(str).truncatedTo(ChronoUnit.MINUTES);
        int minute = ldt.getMinute();
        int remainder = minute % 5;
        if (remainder != 0) {
            ldt = ldt.withMinute(minute - remainder);
        }
        return ldt;
    }
}

Output:

2021-02-08T19:00:00.000
2021-02-08T19:55:00.000
2021-02-08T19:50:00.000
2021-02-08T19:05:00.000