A qualified electronic timestamp is a timestamp issued by a qualified trust service provider that binds data to a specific time. Under eIDAS it carries a legal presumption of the accuracy of that time and the integrity of the data, which is what lets a signature stay verifiable after its certificate expires.
Why signatures need them
Without a trusted timestamp, you cannot prove a signature was made while its certificate was still valid. Timestamps at the T, LT and LTA baseline levels are what give a signature long-term validity (LTV), so it can be checked years later.
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