Java Convert UTC to PDT/PST with Java 8 time library

My program uses LocalDateTime and the value is always in UTC.

A LocalDateTime has no time zone at all, so it is not in UTC.

For a moment in UTC, use the Instant class. This represents a moment on the timeline in up to nanosecond resolution.

Instant now = Instant.now();

To adjust into a time zone, apply a ZoneId to get a ZonedDateTime.

Never use the 3-4 letter abbreviations like PST & PDT so commonly seen in the mainstream media. They are not real time zones, not standardized, and are not even unique(!). Use proper time zone names in continent/region format.

ZoneId zoneId = ZoneId.of( "America/Los_Angeles" );
ZonedDateTime zdt = instant.atZone( zoneId );

It sounds like your data sink has the poor design of taking an input of a string that represents a date-time value assumed to be in America/Los_Angeles time zone but lacking any indicator (no offset-from-UTC, no time zone).

To get such a string, lacking any offset or zone, use the predefined DateTimeFormatter named ISO_LOCAL_DATE_TIME. You will get a string in standard ISO 8601 format like this: 2011-12-03T10:15:30.

String output = zdt.format( DateTimeFormatter.ISO_LOCAL_DATE_TIME );

Your data sink omits the T from the middle, so replace with SPACE.

output = output.replace( `T` , " " );

If your data sink expects only whole seconds, you can truncate any fractional second from your date-time value.

zdt = zdt.truncatedTo( ChronoUnit.SECONDS );

Going the other direction, from string to object, define a formatter, parse as a LocalDateTime and apply the assumed time zone.

String input = "2011-12-03 10:15:30";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuu-MM-dd HH:mm:ss" );
LocalDateTime ldt = LocalDateTime.parse( input , formatter );
ZoneId zoneId = ZoneId.of( "America/Los_Angeles" );
ZonedDateTime zdt = ldt.atZone( zoneId );