Garry Bargsley has a cmdlet for us:
This week’s blog post will help you check your SQL Servers up-time. There are numerous reasons I can think of that you would want to know how long your SQL Server has been online. Was the server recently patched, did it crash and come back online, or did someone restart it by mistake? These are all valid questions about a single SQL Server or your entire estate. I will show you how you can easily check one too many servers quickly for uptime.
We will start by using every DBA’s favorite PowerShell module… dbatools
Admittedly, I’d just check the start time for the tempdb database, but this cmdlet does give more info.