Hi
You can use time based ACL's for this, see the link below :-
PIX
hxxp://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a00804708b4.shtml#tml
IOS
hxxp://www.cisco.com/en/US/products/sw/secursw/ps1018/products_tech_note09186a00800a5b9a.shtml#timebasedtimerange