afl to mark Brexit

I need help to write an indicator which will show significant dates such as Brexit on all charts shown.
I know it's possible to manually draw a line and add text but I want to achieve this automatically.
I have used Metastock and it was possible via their Expert Advisor.
I have tried using something like the code below but end up with arrows above EVERY bar and I'm at a loss.

_SECTION_BEGIN ("_Plot Shape at Date");

IIf( DateNum() == 1160624, PlotShapes( shapeDownTriangle, colorBlack ),0);

_SECTION_END();

Any help would be appreciated

ozbod