Then you have to prove that this was an oversight or mistake but that the mistake does not impact the negotiations or agreement.
Contract law is simple. They teach it first year, first semester in law school
To have a valid contract you need only 3 things
1. An offer by one of the parties (to do something or refrain from doing something).
2. Acceptance by the other party of the offer
These two, taken together, are often referred to as a "meeting of the minds". TO have a contract there must be an agreement as to what will be done.
: There must be an agreement to exchange something of value (time/money/etc) pursuant to the contract.
That is it.
So if you have a written contract where there is an inconsistency, if you can prove that the rest of the contract fairly summarizes the "meeting of the minds"
In the case you describe, it sounds like you had a meeting of the minds of the dates in the body of the contract but that there is an error in the attachment. If that is the case? And you can prove it? Then you can enforce the contract.
It truly is as simple as that
On the other hand if the other party could prove there was no meeting of the minds on the date, the court could void the contract.
So if you want to enforce this? You want to be able to prove you had an agreement...you can do this with the contract as well as with testimony about the negotiations that led to the contract