Afl for Strategy

i want to get the buy sell signals everyday in amibroker

for example

a=macd12
b=macd5
buy=a>b
sell=c<b

sell=short
cover=buy

in this if buy happens today

i want to exit the buy at market close
so we code
exitattime=3.30
sell=short or exitattime
cover=buy or exitattime

i exit at he time mentioned in formula now it turned into short

if market opens gap down tomorrow this sell signal should not act as short for next day tooo
i need fresh short signal

please give me some suggestions my dear seniors

thankyou