According to this site:
You should be able to distinguish using something like:
# Define access time:
acl worktime_am time MTWHF 8:00-11:00
acl worktime_pm time MTWHF 12:00-16:40
# Define access lists:
acl time_unlimited src "/etc/squid/time_unlimited.list"
acl time_limited src "/etc/squid/time_limited.list"
# Set rules:
http_access allow time_unlimited
http_access allow time_limited !worktime_am !worktime_pm
http_access deny all