def initialize(start_transition, end_transition, offset = nil)
@start_transition = start_transition
@end_transition = end_transition
if offset
raise ArgumentError, 'Offset specified with transitions' if @start_transition || @end_transition
@offset = offset
else
if @start_transition
@offset = @start_transition.offset
elsif @end_transition
@offset = @end_transition.previous_offset
else
raise ArgumentError, 'No offset specified and no transitions to determine it from'
end
end
@utc_total_offset_rational = nil
end