We have a standard list of string identifiers that report filters use for their own form elements:

$string['filterfieldoperator'] = '{$a} operator';

$string['filterfieldto'] = '{$a} to';

$string['filterfieldunit'] = '{$a} unit';

$string['filterfieldvalue'] = '{$a} value';

This helps to ensure when the filter type is used multiple times in the same form, that each of it's elements are appropriately labelled and avoid duplication. There are two filters (date and user) that do not use these strings, so let's improve them here to ensure they do (we can reuse the new string from MDL-81488 here)