You may have come across many posts about finding the Installation date of an ESXi, most of them use the first 8 characters of the uuid and convert that to decimal format to obtain the Epoch time and finally converting the Epoch time to human readable time.
Here is also a command line basically doing the same thing but you dont have to go through different sites for convertions. This will do it all for you.
SSH to the ESXi host and use the below command and you should see the Installation date.
date -d @$(printf "%d" 0x$(esxcli system uuid get | cut -d \- -f1 ))
You can also get the Installation date by searching for the installed vib’s. This method works only if you have not patched your server since the date of installation. If you have updated your ESXi after first installation, this command will only give you when the last patch was installed. You can observe the same in my screen below which was run against the same server.
esxcli software vib list | grep 'Install\|esx-base'
Thats because, today i have upgraded my ESXi 6 to Update 2 and thats the latest vib.
Hope this was informative. Thanks!