@DaveMerrett 

So using the flow action you should not need to scrip anything, the convert date to string is built in, you just need to choose it, which I've done when trying to add it to the string field variable, but also have tried doing it directly from a date field to a date field and still doesn't work.

I've checked logs and it shows that it is working, gets the right values, advises of the correct field name, just doesn't actually write to the field it has selected.

As far as my use case:

I have a request type that gets logged that has a preliminary date in it, that needs to be confirmed later by another team that does not have a ServiceNow fulfiller license (and this is the only thing they would potentially need it for).

So I have a second catalog item that that team can use, they input some details about the original request, I do a lookup record based on the criteria, get the record details and update the variable field with the updated/confirmed date, then close off the new request that was logged.

It's just not actually updating the variable in question.