Is the date on a PGP signature reliable?
No. The date and time you see when you verify a PGP signature on a file (often called a timestamp) is the time and date the computer was set to when the signature was created. On most computers, it is extremely easy to reset the date and time to any time you want, so you can generate documents with a forged timestamp. For this reason, you can use a so-called “digital notary” or time-stamping service. This is a system that does nothing but sign documents you send to it, after inserting a date and time somewhere in the text. The service uses a numbering scheme which makes it impossible to insert timestamps at a later time. One such service is run by Matthew Richardson. For more information about it, please see the PGP Digital Timestamping Service website.
