Certificate expiry upcoming Today my phone tells me that its certificate will expire in February 2018

What will happen to my phone when this date occurs?

Do I need (a) to worry (b) to take pre-emptive action? ...
Certificates sign apps as they are submitted to the Play Store, and are controlled by the app developer. If you are getting a warning, it should be specific to the app it reports against. Contact the app developer to seek a rebuilt app with an updated certificate.
The following is from Google's development documentation, and is for the app developer (who should also be aware of the expiration):

Expiry of the debug certificate

The self-signed certificate used to sign your APK for debugging has an expiration date of 365 days from its creation date. When the certificate expires, you will get a build error.

To fix this problem, simply delete the debug.keystore file. The file is stored in the following locations:

~/.android/ on OS X and Linux
C:\Documents and Settings\&lt;user&gt;\.android\ on Windows XP
C:\Users\&lt;user&gt;\.android\ on Windows Vista and Windows 7, 8, and 10


The next time you build and run the debug build type, the build tools will regenerate a new keystore and debug key. Note that you must run your app, building alone does not regenerate the keystore and debug key.

Manage your key

Because your app signing key is used to verify your identity as a developer and to ensure seamless and secure updates for your users, managing your key and keeping it secure are very important, both for you and for your users. You can choose either to opt in to use Google Play App Signing to securely manage and store your app signing key using Google's infrastructure or to manage and secure your own keystore and app signing key.

So, simple answer to:

a) Worry? - Maybe (from further down the documentation page "When your key's validity period expires, users will no longer be able to seamlessly upgrade to new versions of your app.")

b) Action? - note the app and the contact information of its developer, seek replacement app.