This falls into the category of things very few people would notice, but….
Microsoft provides time.windows.com, a public NTP server, operating in stratum 2.
I just came across NTPmonitor, a novel Windows app to monitor a handful of NTP servers. (Sadly, it doesn’t offer the option to sync to any of them, probably because most peoples’ computers let them configure it… Mine syncs to a domain controller which seems to want to give me the time, but not with too much accuracy.)
As with most full-featured NTP clients, it shows you what the remote timeserver reports as its reference clock. I’ve got my server in there, ttwagner.com, showing that it’s currently synced to clock.xmission.com. The “pool” server is pool.ntp.org; whichever of the many machines I connected to is synced to rubidium.broad.mit.edu. On the right we have time.nist.gov, synced to “ACTS,” a NIST protocol.
On the left is time.windows.com, the Microsoft NTP server. Its upstream timeserver?
Screenshot attached, since I wouldn’t believe it without one.