The expiration time is checked by the client. If you want to turn checking off, you have to modify the client. But this is what you wanted to avoid.
So the best thing you can do right now is to create a new certificate, this time with a long expiration time, e.g. 100 years. Matthias ________________________________ This electronic message, including attachments, is intended only for the use of the individual or company named above or to which it is addressed. The information contained in this message shall be considered confidential and proprietary, and may include confidential work product. If you are not the intended recipient, please be aware that any unauthorized use, dissemination, distribution or copying of this message is strictly prohibited. If you have received this email in error, please notify the sender by replying to this message and deleting this email immediately.