Georgius Cedrenus − CSHB9: 484-5: "Nona Novembris die, indictione 12, anno 6537, subito morbo correptus Constantinus... triduo post Constantinus vivendi finem fecit, cum vixisset annos." The Latin translation indicates that he fell ill on the 9th and died 3 days later. However, the text could also be interpreted as the third day since his illness, that is, 11 November.
Lupus Protospatharius; 1029: "mortuus est Constantinus Imperator in vigilia S. Martini." On the eve of Martin of Tours', that is, the day before 12 November (unless it's referring to the very same day).