LocalDateTime add millisecond

Question

I want to increase the millisecond value with LocalDateTime. I used plusNanos because I didn't have plusmillisecond. I wonder if this is the right way. I'm using JDK 1.8. I also want to know if there is a plus millisecond function in later versions.

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");
LocalDateTime ldt = LocalDateTime.parse("2022-01-01 00:00:00.123",f);
        
System.out.println(ldt.format(f));
        
ldt = ldt.plusNanos(1000000);
        
System.out.println(ldt.format(f));

2022-01-01 00:00:00.123
2022-01-01 00:00:00.124

Answer 1

Adding nanoseconds is a completely valid way of doing this. If you want a solution that is more self-explanatory, you can use LocalDateTime#plus(long, TemporalUnit):

private static final DateTimeFormatter FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");

public static void main(String[] args) {
    LocalDateTime localDateTime = LocalDateTime.parse("2022-01-01 00:00:00.123", FORMATTER);
    localDateTime = localDateTime.plus(1L, ChronoUnit.MILLIS);
    //         How much you are adding ^              ^ What you are adding

    System.out.println(FORMATTER.format(localDateTime));
}

The TemporalUnit parameter explains exactly what you are adding to the the timestamp, thus making your code more easily understandable to other programmers who may be viewing it. It also takes care of the unit conversions behind the scenes, so there is less margin for human error and your code is not cluttered with math.

Answer 2

The Answer by Cardinal System is correct.

Duration

An alternative is use of Duration class with LocalDateTime#plus.

A Duration object represents a span of time not attached to the timeline, on a scale of hours, minutes, seconds, and fractional second.

LocalDateTime ldt = LocalDateTime.parse( "2022-01-01 00:00:00.123".replace( " " , "T" ) ) ;  // Replace SPACE with `T` to comply with standard ISO 8601 format.
Duration duration = Duration.ofMillis( 1L ) ;
LocalDateTime later = ldt.plus( duration ) ;

---

By the way, be aware that LocalDateTime is inherently ambiguous. That class represents a date with a time-of-day but lacks a time zone or offset-from-UTC needed to determine a point on the timeline.

For a specific point on the timeline, use Instant.

Instant instant = Instant.now() ;
Instant later = instant.plus( duration ) ;

To see that same moment through the lens of a particular time zone, apply a ZoneId to get a ZonedDateTime object.

ZoneId z = ZoneId.of( "Pacific/Auckland" ) ;
ZonedDateTime zdt = later.atZone( z ) ;

See this code run at Ideone.com. Note the differences: (a) one millisecond later, and (b) different time-of-day & different date.

instant.toString(): 2023-03-29T19:12:16.727887Z
later.toString(): 2023-03-29T19:12:16.728887Z
zdt.toString(): 2023-03-30T08:12:16.728887+13:00[Pacific/Auckland]

If you want only milliseconds, you can truncate to clear the microseconds & nanoseconds to zero.

Instant trunc = instant.truncatedTo( ChronoUnit.MILLIS ) ;