Legal Question in Elder Law in Pennsylvania

I am not sure what you are asking as your question makes no sense as phrased.

There are all kinds of legal documents - deeds, wills, trusts, powers of attorney, contracts etc.

All are "legal" documents. I don't know what an illegal document would be - I guess it would depend on what the document says. For example, a document purporting to be a receipt for the sale of a relative would be illegal since slavery is outlawed in the US.

I assume therefore that your question is relating to validity of the document, rather than its legality. Dating a document may or may not affect its validity. I would need to see the document to know what kind of document it is. For example, there is no requirement that a will be dated. It just needs signed at the end by the testator and witnesses. However, most wills have dates if they are done by lawyers. But if this is a handwritten will, then it may not. But that does not make the will more or less valid if it was properly signed and witnessed.

So you need to have the document reviewed by an attorney to see exactly what it is and whether it meets the requirements in order for the document to be valid without the date on it.