A timestamp proves the existence of a certain document at given point in time; more specifically that the particular document already existed before the moment when the timestamp was affixed. The document is represented in the timestamp by its electronic imprint which is obtained through certain mathematic function from its content (hash code) and is unique to each document.
A timestamp can be obtained for each individual document. However, it is more economic to stamp a batch containing multiple documents, such as on a daily basis, provided that such documents are stored together in a particular archive repository.
The timestamps also ensure document integrity, i.e., their validity proves that the timestamped document has not been modified in any way since the timestamp was issued. Any change to the timestamped document would result in the invalidity of its timestamp as the document modification would result in change of its digital imprint (the hash code).