ERR_CERT_DATE_INVALID on iPhone
Why it happens on iPhone
iOS trusts the certificate against the device clock. A phone whose date drifted, or one restored from an old backup, reads a perfectly valid certificate as expired or not yet valid.
The fix on iPhone
Settings → General → Date & Time → Set Automatically, then reload. If the date was already correct, the certificate really has expired and only the site owner can renew it.
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
iPhone 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