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Increasing numbers of documents are currently being stored in electronic archives. These documents are often digitally signed to ensure non-repudiation. However, the term of validity of digital signatures is usually only 2 years, so the archive must extend their validity to the required storage period. The simplest solution to this problem is to use XAdES-A specification and process each signed document separately. Such an approach requires the creation of a timestamp for each signed document. This time stamping operation is very time consuming; therefore several improved techniques have been proposed, for example, evidence record syntax (ERS). The documents in an archive are often related to one entity. The ERS allows us to prove the existence of a document group in one round. This study presents a solution extending ERS and providing a functionality that enables proof of the existence of a document group over a long period (multiple rounds).