IBM has provided the program QWCCRTEC, which will generate a report of activity on your last IPL. From a command line enter the following command:
CALL QWCCRTEC

This command creates a spooled file named QPSRVDMP. It is a hexadecimal dump of the system's last IPL information. If you scroll to the right side of the report, you can see the character representation of the hex codes. You will see the SRC (System Reference Code) that represents the IPL step and the date Date and Time of the IPL step.

Here's a snippet of the report:

 
*XPF IPL        06/20/05 08:45:08* 
*  C900 2810    06/20/05 08:45:08* 
*  C900 2820    06/20/05 08:45:08*
 

To understand the SRC Codes such as 'C900 2820', you can review the IBM SRC finders.

V5R2 SRC Code Finder:
http://publib.boulder.ibm.com/iseries/v5r2/ic2924/index.htm?info/rzamf/rzamfiplsrcfinder.htm

V5R3 SRC Code finder:
http://publib.boulder.ibm.com/infocenter/iseries/v5r3/ic2924/index.htm?info/rzahb/rzahbiplsrcfinder10.htm