The most likely cause of certificate warnings across various secure websites is incorrect date and time settings on the user's computer. Secure websites rely on SSL/TLS Certificates, which have a specific validity period. If the system clock is set to a time outside of the certificate's validity period, the browser will display a warning. This behavior is by design to prevent man-in-the-middle attacks and other forms of security compromise. The user receiving certificate warnings on several well-known websites strongly indicates a problem with their system rather than with the certificates of those individual sites. Adjusting the date and time settings to the correct values should resolve these warnings. A single expired certificate or a weak encryption cipher would not cause warnings on multiple sites that likely use different certificates and encryption methods.