A computer uses the information included within a digital certificate to obtain a current date and time value from a trusted source extrinsic to the computer. The computer requests and receives the trusted current date and time value and compares the trusted current date and time value to a validity period included in the digital certificate, to determine if the digital certificate is expired. The information included within the digital certificate specifying an extrinsic source for the current date and time value can be included in an extension of the digital certificate, and the information can specify a plurality of extrinsic sources.