6.1.c Implement and troubleshoot logging

6.1.c [ii] Timestamp

System log messages can contain up to 80 characters and a percent sign (%), which follows the optional sequence number or time-stamp information, if configured. Messages appear in this format:

Seq no:timestamp: %facility-severity-MNEMONIC:description

The part of the message preceding the percent sign depends on the setting of the service sequence-numbers, service timestamps log datetime, service timestamps log datetime [localtime] [msec] [show-timezone], or service timestamps log uptime global configuration command.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Location 5585).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/fundamentals/command/Cisco_IOS_Configuration_Fundamentals_Command_Reference/R_through_setup.html#wp4972384860