If the date of an incident you have received a written warning for is wrong , does this make the warning null and void

No, of course not, you may ask for it to be corrected.

Written warnings are not like law suits but even traffic tickets are not void, if there are minor information mistakes.

Warnings are for company policy, or union contract. In many companies, they are not even required.

How a company deals with employment problems is totally based on company policy. There is little if any law govening this. So nothing is going to make a warning void, especially not a minor mistake. Company warnings are not a court of law.

To easy to correct a date as long as the details are correct. That's what you go by, the details of the text.