Timezone conversion issues in PowerQuery

I've figured it out! 
It's because NZ summer wraps around the new year. 

So I need to calculate WinterTime instead and adjust the calculation like so: 

(datetimecolumn as datetime) =>

let

date = DateTime.Date(datetimecolumn),
time = DateTime.Time(datetimecolumn),
ForwardDate = Date.StartOfWeek(#date(Date.Year(date), 9, 30), Day.Sunday),
BackDate = Date.StartOfWeek(#date(Date.Year(date), 4, 7), Day.Sunday),

isWinterTime =
		(date = BackDate and time >= #time(2,0,0))
	    or
		(date > BackDate and date < ForwardDate) 
		or 
		(date = ForwardDate and time < #time(2,0,0)),

timeZone = 13 - Number.From(isWinterTime),

NZ_time = 
            DateTime.From(date) 
            + #duration(0,Time.Hour(time),Time.Minute(time),Time.Second(time))  
            + #duration(0, timeZone, 0, 0)

in
    NZ_time