How to get seconds from LocalDateTime?

Question

Java

Good afternoon, how can I convert a variable into seconds of int or long format?
Basically I need to take two timestamps subtract and get elapsed time in seconds.
UPD. The question is closed.
decision

ZoneOffset offest = ZoneOffset.UTC;
       Long endSessionSeconds =  endSession.toEpochSecond(offest);
       Long startSessionSeconds = startSession.toEpochSecond(offest);