Hi
You can try using the Time Range feature available with IOS which can be binded along with the access-list.
You can deny the access to your PC-B to outside world and permit all kinda access to your PC-A conditioned to the Time range which you define under the time range group.
for more info i would suggest to check this link ..
http://www.cisco.com/en/US/products/sw/secursw/ps1018/products_tech_note09186a00800a5b9a.shtml#timebasedtimerange
but do make sure that you are setting the clock and timezone properly,also if possible do sync the router with the external time source using NTP.
You may see undesired results if the router is rebooted and the clock is reset back to the default.
in this case the time range wont work and may block all traffic ..
regds