Verify the timestamps of the signed document
When you check the signatures of the PDF document each signature may include one or more timestamps. To get the number of timestamps made over the signature check TimestampCount property of the security handler object, assigned to the signature. The object is of TElPDFPublicKeySecurityHandler type and is present in the Handler property of TElPDFSignature class.
Timestamps property returns objects of TElClientTSPInfo type, which contains all information about the timestamp. If the timestamp is not valid, it's Time property is equal to 0 in VCL edition or to 30 December 1899 in .NET edition (to check, use Time.ToOADate() = 0 expression).