Display fields from a paragraph as a single field I have a paragraph which is essentially a date range, however, the individual dates can have prefixes, suffixes, can be in differing formats, I have a dropdown to select the format and dates need to be displayed differently depending on whether they span within a decade, over decades or over century boundaries. The date values have been split out into a daterange field so we can search / filter by date, however, within the parent paragraph, I'm wanting to return just the processed date instead of the list of fields.

I can do the IF this RETURN xyz to get the date fields into the required formatted string, however, I'm struggling to find how I can build this into a plugin and attach to my desired paragraph(s) / nodes.

It's fairly simple to do this within the theme templates, however, I'm needing to do this at a field / node level, not a twig template as the data will be pulled both as a traditional web page, but will also need to be pulled out via XML export, probably within various views and so on, so don't want to be doing the presentation transformation in multiple places. I need it to be independent of the site theme files.

This is in Drupal 8.8

Hope this makes sense.
If you wish to have absolute control on the display of your date range, I'd recommend you to create a module with a new FieldFormatter to expand one of the DateRange fieldformatters (the original files are in code/modules/datetime_range/src/Plugin/Field/FieldFormatter).

By overriding the viewElements() function, you'll be able to set the desired display(s) for your date range from one place and independent from the site theme files.

Once your new fieldformatter file is created, go to your paragraph "manage display" page and modify the display of your date range field to your new fieldformatter.

Hope this helps.