Are you restricting traffic on the inside interface of your pix ?
If so you will need to allow through requests to an NTP server
object-group network servers
network-object host 192.168.5.10
network-object host 192.168.5.11
object-group network NTP_servers
network-object host x.x.x.x
network-object host x.x.x.x
access-list inside_out permit udp object-group servers object-group NTP_servers eq 123
This will allow outbound requests to external NTP servers. The return packets should be allowed in because of the stateful nature of the pix (pseudo-stateful in terms of UDP).
If you are not restricting outbound access then you shouldn't need to do anything.
Note - the above assumes that you have all your NAT setup correctly.
Jon
