ERR_CERT_DATE_INVALID on Android
Why it happens on Android
Android's certificate store is updated with the system, so an unpatched device rejects certificates issued by CAs it has never heard of and reports them as date-invalid after the old root expires.
The fix on Android
Set date and time to automatic, then update Android System WebView and Chrome from the Play Store. On Android 7.1 or older, expired ISRG/Let's Encrypt roots cause this permanently — the site must serve an alternate chain.
Is ERR_CERT_DATE_INVALID happening on your domain?
The SSL checker returns the exact notAfter date and days remaining for the hostname, which immediately separates 'genuinely expired' from 'your clock is wrong'.
Free, no signup. Runs from outside your network, which is the only way to tell a server fault from a local one.
If that didn't fix it
Android has its own failure mode, but ERR_CERT_DATE_INVALID has a wider set of causes that apply everywhere. The most common one overall is: the certificate expired and auto-renewal silently stopped.
All causes and fixes for ERR_CERT_DATE_INVALID