How to code Weekly Range indic to include Sundays?

Hi, I am learning to code, but just doing the very basics right now.

I have just noticed that the weekly range does not start on Sunday.

My code is a simple one,

zRange = W1_High - W1_Low ;

So this calculates the weekly range, but it apparently starts on Monday. How do I code it to start on Sunday? I live in the US so FX trading here starts on Sunday, and am trying to code an indi that keeps the weekly range updated. My code did not roll over to the new week.

Thanks

Regards
Joe

Sorry, forgot to add what W1 High and Low stand for,

W1_High = iHigh(NULL,PERIOD_W1,0) ;
W1_Low = iLow(NULL,PERIOD_W1,0) ;