The issue is: You are concatenating direct DateTime field to the string, but you can only do that for String or integers.
Use https://docs.mendix.com/refguide/parse-and-format-date-function-calls#5-formatdate-utc
So you must do: formatDateTime( $NewInterview/StartDateAndTime, 'yyyyMMddHHmmss' )
NOTE: you can have any format that Java SimpleDateTimeFormat supports. Read https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/text/SimpleDateFormat.html