The operating system my web server runs on is (include version): Windows Server 2012
For most the cert is valid, but a few get NET::ERR_CERT_DATE_INVALID. It's happening for example one someone's new iPhone 12, both on safari and chrome.
Which typically means you are using the cert.pem file instead of the fullchain.pem file.
But because you are using Apache on Windows Server 2012...
I can't be altogether certain on how you are obtaining certs nor their file names.
But the basic idea remains in place - it needs to serve the cert + the chain (not just the cert).