Update the date field to the date & time of the node being updated
Nodes already have a 'changed' property that would track this. Any reason why you don't want to use that? You can use {{ node.getChangedTime() }} in a Twig template for the raw value, or you can preprocess the node and take the changed value and format the date to your need into a new variable for the template. This way you don't need another field and Rules and reduce a lot of overhead.