Hello,
You can use:
time() - hour(vector(time())-8.5*3600)*3600 - minute(vector(time())-8.5*3600)*60 - time()%60
If you want only at that time, you could try, so it will be firing for 5 minutes every day at 8.30:
time() - hour(vector(time())-8.5*3600)*3600 - minute(vector(time())-8.5*3600)*60 - time()%60 and time() - hour(vector(time())-8.5*3600)*3600 - minute(vector(time())-8.5*3600)*60 - time()%60 - time() > -300
Regards,