I was investigating some VM issues recently, and needed to find out when an ESXi host last rebooted to see if I could correlate logs with events to discover the source of the issue.
There are a few of easy ways to find out the last time an ESXi host booted, there are probably more, but these should be enough for a quick check.
Firstly in vCenter, on the left hand side browse to the top level, the Datacenter level or Cluster level. Then in the right hand pane look at the hosts tab, and scoll along to the uptime column.
The second and third methods involve connecting to the ESXi host via SSH. For this I use Putty (see the Useful Tools page).
Once connected to the ESXi host you can use the command uptime to see the last time an ESXi host booted, or you can use the command grep booted /var/log/vmksummary.log to view historic reboots if you have persistant storage for logging.