What is the difference between actum and datum in diplomatics?

In the field of diplomatics, the terms actum and datum refer to two distinct temporal points often recorded within a document to establish its history and legal validity.1

• Actum: This refers to the date and place of the actual legal enactment or transaction itself. It marks when the event—such as a gift or decree—was performed, often orally or in a preliminary fashion.12
• Datum: This refers to the date and place of the document’s issuance. It records when the physical document was created or finalized to serve as a record of the previously performed legal act.1

Distinguishing between these two is essential for scholars, as the form and method of dating are critical factors in determining the provenance and authenticity of a charter or other historical record.1