The date before 887 is suggested by Æthelflæd's name being attached to the charter quoted here. (A date of 880 is attached to the charter, but Baldwin notes that indiction suggests a date of 887. Baldwin expresses some doubt that the charter is genuine.)
Bailey accepts the charter evidence: "The marriage had definitely occurred by 887, when Æthelflæd signed the diploma S 217 as coniunx (Sawyer 1968:125)."