The decoder default for leap seconds is 18 seconds when using GPS time.
GPS time was set to "zero" at 00:00:00 on 1980-06-01, subsequently GPS time is now ahead of UTC time by 18 seconds as there have been 18 leap seconds since this date.
When the leap seconds value is updated we will include this in future firmware revisions.
The only time it may be necessary to modify the value for leap seconds is when using third party hardware alongside your race result decoder, other hardware may not update leap seconds to the same value.