Yes, as others have stated, the freedom to contract allows you draft an agreement that has a retroactive effect (unless it is against public policy for some reason). It might be smart to include a short explanation within the NDA that addresses why the agreement covers a former time period. This would help clear up any confusion as to why the dates look funky upon initial glance.
It certainly wouldn’t hurt for an attorney to draft or review your NDA. Skilled attorneys know what to look for to limit (and hopefully eliminate) the risk for IP infringement and/or breach of contract far better than the standard NDAs you can find online
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