If when you run show logging or try to view the debug logs on a Cisco IOS device the date and time is not displayed and instead you get something like the below, here is the solution.

6w4d: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on FastEthernet2/0/8.

Get into configure terminal mode and enter the below

service timestamps log datetime localtime
service timestamps debug datetime localtime

This will get the Cisco IOS device to record the local time with each entry into the log, so you get something like the below

Jul 29 09:32:51: %SYS-5-CONFIG_I: Configured from console by vty0

You will also want to make sure the date and time is correct on you device otherwise the date and time in the logs will be off.

show clock
09:40:12.581 BST Thu Jul 29 2010

And if you need to set the time

clock set hh:mm:ss

or

ntp server xxxxx

All future logs will not have the date and time on them.