I was wondering if I'm seeing a bug or an oversight on my part. I was trying to type a $date as xs:dateTimeStamp but getting an error using the BaseX GUI, v8.6.7.

let $date := '2017-12-15T23:45:00-5:00'
return
  xs:dateTimeStamp($date)

I've read in Walmsley's that XSD v1.1 data type support is required for xs:dateTimeStamp. Is this a bug or maybe user/operator error?