LinuxLogFile format in Ubuntu 8.04

[Douglas Gilbert]

I can find no explanation of the proforma part of log file
entries.

Here is a typical entry from /var/log/messages:
Jun 19 10:17:23 rink kernel: [   25.352589] agpgart: Putting AGP V2 ...

Where is that format defined (from the start of line to
'agpgart:' in this case)? Why was the square bracket enclosed
section added? What is it? [Seconds since boot up?]. What tools
can decode what is in the square brackets? Can the proforma part
of a log entry be user configured?.

The LinuxLogFile document could be enhanced by adding a section
to explain the log file output format.

TIA
Doug Gilbert