ERR_CERT_DATE_INVALID on Cloudflare
Why it happens on Cloudflare
The Cloudflare edge certificate is valid but the origin certificate behind it expired, so Full (Strict) mode fails on the origin leg.
The fix on Cloudflare
Check the origin directly by IP with an SSL tool, not through Cloudflare. Renew the origin certificate, or replace it with a 15-year Cloudflare Origin Certificate that cannot silently lapse.
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
Cloudflare 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