If you are requesting the date of a medical certificate to be in the past, this is what you would traditionally call "backdating", which is not in compliance with the guidelines in Australia.

For example, if you receive a consultation on the 25th of July, but you want the certificate to state that this was provided on the 24th of July, this is not allowed.

If, however, you consider a scenario where you request a medical certificate consultation on the 25th of July, and you were too sick on the 24th of July to make the request. If your doctor deems it suitable, they can provide you with a medical certificate that covers the medically justifiable period up to a day prior to the requested date.

This means you could be covered for the 24th and the 25th of July. The date the certificate is generated can only be from the date it is requested, which would be the 25th of July in this case.