I do not know what the process is QB Online is, but assuming it is functional at all, the post-dated checks should not have anything to do with the reconciliation.
When you reconcile a bank account, the only items that matter are those that have cleared, after all.
As far as your apparent account balance being off, well, that's just how it is. As far as your company is concerned, that money is no longer theirs regardless of whether they have been formally cashed or not.
