Generally, no documents should be backdated. However, if it is an agreement that was effective before the document was signed, or it was recording action that had already been undertaken but not documented, there can be a 'with effect form [date]' that is earlier than the date of signature.
Backdating to mislead is a slipperly slope. One company I dealt with backdated a document to be earlier than the signatory joined the company, which destroyed the credibility of its whole case