Will a worshiper who prayed all prayers before their prescribed time by more than a year due to a mistake in their phone's timing, and only just found out, be held accountable?

The one who errs is not held accountable, according to the Hadith: "Indeed, Allah has overlooked for my Ummah (nation) their mistakes, forgetfulness, and what they are coerced into." As for prayers that were performed before their due time unintentionally, they are not valid according to the majority of scholars, and must be re-offered.