It is not a good but a simple idea to generate an evaluation for a trial of some software by signing on the evaluation information. For example expired date will be the lock-field of the signature. Because the signature could not be cracked, so it's for sure the evaluation information won't be modified! But as the procedure of verifying signature is running on client, client would always hack the software's source to skip the verifying. So after all it is not a good idea but a simple idea for doing so.