Fix - SimplePie Errors
A WordPress plugin that adds a temporary fix for an incompatibility issue of the built-in library, SimplePie 1.3.1, with PHP 7.1 or above.
Getting PHP Errors?
PHP Warning: A non-numeric value encountered in .../wp-includes/SimplePie/Parse/Date.php
For details, see here.
So until the issue is fixed in the core, this plugin can be used to avoid the warnings for the time being.
Just activate the plugin. And the warnings should be gone.