Can an entirely incorrect term void a lease

Probably not in this instance.  The contract is whatever the written lease says, particularly since it probably has a merger clause providing that the writing represents the entire agreement and supercedes all prior oral discussions.  It is possible (but unlikely) that, if you can show that the lease was intended to be for 6 months and both sides mistakenly thought that six months would take it to March31, that you could get a court to reform the lease so that it ran until April 6, but the lease would not be void.