Recently, I finally got around to managing my server (which I've neglected for a while since switching from Uverse to Xfinity... another post on that later). When I updated my BIOS settings to be the proper date, my ESXi server started complaining about an expired license.
To give some background, I've been using an ESXi vSphere enterprise plus license key that I got for free through my university's software program. Supposedly, I was supposed to renew my license through the university program vendor every year, but I ended up never having to, for some reason, and my license stayed valid even though it was expired. See picture below:
Given that the only thing that changed from before was that I updated my BIOS date which invalidated my license, I guess that ESXi was somehow basing the license validity off of my BIOS date rather than some NTP server. I ended up changing my BIOS date to a few years before the expiration date and it worked!
I've never seen a DRM system in the wild that bases it's time off of the BIOS time, but I guess this is a useful tidbit for those who have an expired license key with more features than the free tier license key that VMware has given to the community.