It would be very easy for me to try and say what a reliable and trustworthy sort of a fellow I am and that you can naturally assume that Stamper will provide accurate timestamps which will never be backdated. This would not be good enough and, indeed, Stamper does better.
Every signature made by Stamper will have a unique serial number. This number automatically increments by one every time something is signed. All signatures made by Stamper are public and may be inspected by anyone. However, what has been signed is not public, nor is it retained by Stamper other than temporarily for debugging purposes.
To demonstrate trustworthiness, Stamper timestamps all of its signatures on a daily basis. It also stamps its daily summaries on a weekly basis. Thus, it would not be possible for any signatures to be backdated, without breaking this chain of signed signatures. This feedback mechanism is similar to the techniques used in more modern blockchain technologies.
Thus the combination of the serial number being in chronological order coupled with publishing the signatures should provide sufficient certainty that timestamps have not been back (or forward) dated.