Shifting event times to and from time zones
You can add an argument of type:

BiFunction&lt;LocalDateTime, Integer, LocalDateTime&gt;


To your function that will reflect your need: LocalDateTime (initial date), offset, LocalDateTime (date that will be returned after applying the function). 
The method will look like this:

private void shiftEventDateTime(EventDto event, BiFunction&lt;LocalDateTime, Integer, LocalDateTime&gt; f){}


To apply the function inside the method you can write it this way:

f.apply(event.getStartDateTime(), getOffsetSeconds(event.getStartDateTime(), event.getStartDateTimeZone()));
f.apply(event.getEndDateTime(), getOffsetSeconds(event.getEndDateTime(), event.getEndDateTimeZone()));


And when you invoke your method you need to specify the operation itself:

shiftEventDateTime(event, LocalDateTime::minusSeconds);
shiftEventDateTime(event, LocalDateTime::plusSeconds);


You can choose whatever name for the BiFunction that you like and the one that fits your case the most.