Yes, it would be valid. The date was filled in by the notary, so it is known when it was executed. The failure to include the date on the first line therefore is a "harmless error" that does not affect the validity or reliablity of the amendment and so does not affect its enforceability. Only errors or omissions that fundamentally undermine knowing whether the agreement was properly executed or understanding what was intended or when it goes into effect would render it invalid, but this omission does not do that.