Learning AmiBroker

Hi,

I am Learning AFL and need some help.

Can someone guide me for the following:
I am calculating Opening 15 minute High and Low. And want to trade rest of the day using those values. But in Day 2 and so on, for first 15 minutes, these variables value should be 0. Currently, it is retaining previous day's values. How can I reset it?

Here's part of my AFL:
TimeFrameSet( in1Minute );

tn = TimeNum();

ObservationStartTime = 91559;
ObservationEndtime = 92959;

StartBar = tn == ObservationStartTime;
EndBar = tn == ObservationEndtime;


Opening15MinHigh = ValueWhen( EndBar, HighestSince( StartBar, High ) );
Opening15MinLow = ValueWhen( EndBar, LowestSince( StartBar, Low ) );


Please guide how can I achieve this?

Thanks

Learning AmiBroker