Is your feature request related to a problem? Please describe.
My use case is (again) moving from one instance to another, but I can also think of backup : I'd like, like the others fields, to export and import the field used in interpreting the date when scraping XHTML+CSS

Describe the solution you’d like
A new field in OPML file called frss:TimestampCustomFormat

Describe alternatives you’ve considered
None (I don't admin my own instance)

Additional context
I wold be happy to provide more