Go to http://www.allegro.com and click on the "UPTIME" link under "Free Software".

Print the contents of 'LOGDCC.PUB.SYS'

The creation date of the files in the group BOOTUP.SYS is the last START NORECOVERY and the modification date is the last START RECOVERY.

SYSINFO.PRVXL.TELESUP, under the MISC option, displays the last install, last update, last start and possibly last dump.

[Editor's note: All the suggestions work though I lean toward SYSINFO because of its completeness and presence on all systems.]