This was a suggestion from a call to review the SPDX 3.0 spec with BSI.
The following regex would produce a more specific verification of the date time format:
^\d\d\d\d-((0[1-9])|(1[0-2]))-((0[1-9])|([12]\d)|(3[01])T(([01]\d)|(2[0-3])):[0-5]\d:(([0-5]\d)|60)(\.\d+)?Z$