Notary public Is is valid

It may allow it to be challenged, it is not invalid just because of this, the agreement would have been written on a specific date, but not signed till another date a few days latter.

It would help to know what type of contract it is,
Since of course was the contract acted on, was other agreements done after this to, to change the terms and so much more

I agree with Fr_Chuck - quiite often a contract is dated as effective on a certain date, but with multiple parties having to sign and the delays of sending the document via mail from one party to the next the actual signature dates are quite often different.

This would not be considered invalid as the notary probably had the correct date when he notarized the document. The date the notary always uses is the date the notary witnessed the signatures - not the date contained within the document itself. The notary is only attesting to the fact that the individual signed the document on such and such date in front of him and nothing else. This in itself does not invalidate the document.

Lots of times documents are made up days or even weeks in advance and the individuals who sign this may not have been available the date the document was prepared. Happens all the time. I've notarized documents that had dates as much as a month prior to the individual signing it.

The wording of the notary is clear. The notary is not saying they witnessed the signature. They are saying that the signator was known to them and attested to signing the document. The notary may not have witnessed the sgnature, but is verifying that the signator acknowledged signing the document.

Thank you for your replies. It is now very clear to me.