Backdating in most circumstances is considered fraudulent and illegal. For instance, backdating a claim for a past period is taken as legal.
Likewise, if you haven’t made the loan yet, you may set the effective date to a future date, so that the interest only starts accruing when the actual loan has been made.
The technical aspects of backdating a mail are discussed in the forum thread below.
As you will see, it is possible, but no computer savvy person will be fooled because on its way it will receive electronic time stamps which cannot be altered.
In other words, if you sign that loan agreement today and its effective date is a month from now, you are bound to the agreement starting today even though you will not begin acting on it for a month.
Sometimes courts are asked to determine the effective dates of contracts that are completely undated.