If you have an SSL certificate, whether it be for a secure website, an SSL VPN tunnel, or what have you, it's going to have an expiry date.
If you let this certificate expire, it may result in anything from scary warnings to your website visitors, through to your VPN ceasing to work.
An easy way to check the expiry date of your certificate, on either Linux or Windows, is to use OpenSSL. This is usually included in most Linux distributions, but it's a separate free download for Windows.
To display the expiry date, use this command:
openssl x509 -noout -in <certificate file> -dates
Of course, this can easily be scripted on either operating system, and the results emailed or texted to you.
Keep on top of your security certificate expirations. It's important stuff.