Validate signature without date

By "electronic signature" do you mean a digital signature (backed by cryptographics algorithms and public/private key pairs) or some arbitrary scribble?

You speak about validating. That sounds like you have a digital signature.

But then you say that a field for date not filled in keeps you from validating. That does not sound like a digital signature. Unless, that is, you mean by field for date not filled in that there neither is the M entry in the signature dictionary nor the signingTime signed attribute nor a signatureTimeStamp unsigned attribute in the CMS signature container nor a document time stamp covering your signature.

Such a digital signature would not be per se invalid, merely of limited usefulness. You would generally validate such a signature at the current time.

A digital signature contains the date it was applied automatically. Anyway, if the file has been locked by the signature you can't edit its contents any longer. If something is missing you need to ask the person who signed it to fill it in, sign it again, and re-submit it.