Process first and last dates.
Arguments
- start_date
chr: The earliest date (ymd) search results should be returned. Cannot be used with days_back.
- end_date
chr: The latest date (ymd) search results should be returned. Cannot be used with days_back.
- days_back
dbl: Length of time the search extends in days since the document was published on ADAMS? Default is all time (i.e, NA). Cannot be used with start_date or end_date.