If you need information about the last time your DB2 instance was started, you can perform the following command:

db2 get snapshot for dbm | grep -i manager

The output result you'll get should be something like this:

Start Database Manager timestamp               = 03/17/2008 07:00:41.018041

In addiction, if you also want to know when was the last time your server was rebooted, you can perform on AIX command prompt:

$ who -b

The output for this command should be something like:

   .        system boot Jan 27 04:19