Complex sentence: The document probably dates from after 914 during the reign of Alfred's son, Edward the Elder, this assumes that it was compiled as part of the preparations for Edward the Elders campaign against the Danes in 917.
Split into simpler sentences: The document probably dates from after 914 during the reign of Alfred's son, Edward the Elder. This assumes that it was compiled as part of the preparations for Edward the Elder's campaign against the Danes in 917.