Proving creation time/date of a screenshot

If you want to prove to others that you took the screenshot on a specific date and not later, you will not be able to do it yourself, you will have to rely on some common trusted third party.

For low importance issues, this can be accomplished by simply posting the image on some well known public service where the date when the image has been posted will be mentioned. Be sure to check beforehand that this service does not offer any possibility to modify the picture without altering the upload date!

For higher importance issues, you will have to contact a bailiff or a notary. By being present during the screenshot, they will be in measure to vouch the date and the conditions when it has been taken. For instance I've read about such procedure being used when someone wants to keep a proof of the existence of a security flaw still valid even once the flaw itself has been corrected. However, if you go that way I would strongly recommend to check the Law StackExchange website before engaging yourself into anything.

Uploading a hash of the screenshot to the Ethereum blockchain allows anybody in the future to make sure that the screenshot was taken before the hash was uploaded. Of course, you should choose a hash function that is cryptographically secure (and in particular, collision-resistant).

If you however also need to make sure that the screenshot wasn't taken before a certain date you have to go to trusted person like a notary to have them create the screenshot.

If you don't need complete legal certainty, the Wayback Machine has a "Save this url in the Wayback Machine" feature. The Wayback Machine doesn't gurantee their own data integrity but their data is relatively trustworthy for many applications. archive.is is another similar tool.

Ask a mutually-trusted third-party to use their own independent equipment to make the screenshot and ask them to date and sign it (digitally or using ink)

The normal way is to pay a solicitor/lawyer/notary to do this.

It is trivial for you to fake a date in image metadata (EXIF etc) or in a digital signature - so you need certification by someone that will be trusted by anyone with doubts who you need to convince.