You can do this ..
try this
# This line will set the acl "eve" to donate time between 1800 to 20:30 hr.
acl eve time 18:00-20:30
#these are files extension u want to stop from downloading, you can add more.
acl bad url_regex -i \.exe$ \.mpg$ \.avi$
# now this line will deny those files when its eve ie evening.
http_access deny bad eve
# now we have to allow bad rest of the time
http_access allow test
Try above, its not tested, and I hope you have got some idea.