Java8 Adding Hours To LocalDateTime Not Working

The documentation of LocalDateTime specifies the instance of LocalDateTime is immutable, for example plusHours

public LocalDateTime plusHours(long hours)

Returns a copy of this LocalDateTime with the specified number of
hours added.

This instance is immutable and unaffected by this method call.

Parameters:
hours – the hours to add, may be negative
Returns:
a LocalDateTime based on this date-time with the hours added, not null
Throws:
DateTimeException – if the result exceeds the supported date range

So, you create a new instance of LocalDateTime when you execute plus operation, you need to assign this value as follows:

LocalDateTime nextTime = currentTime.plusHours(12);
Instant instant2 = nextTime.toInstant(ZoneOffset.UTC);
Date expiryDate = Date.from(instant2);
System.out.println("After 12 Hours = " + expiryDate);

I hope it can be helpful for you.