There is no way to broadcast a transaction today which will be scheduled to be mined at a future date. I suppose you could set up some software to broadcast a transaction for you at a set date, but then you run the risk of the software not working, or the computer dying, or going offline, or anything else, which you wouldn't be around to fix. The best thing you can do is to sign a transaction today which cannot be mined until a set date, and then share it (or where to find it) with a third party for them to broadcast in the event you can't.

I can make a transaction moving all my coins to your wallet, and say that it cannot be mined before block 800,000. I can then either store it my safe, knowing that as the inheritor of my estate you'll be able to access it after I die, or I can send you the signed transaction to keep yourself, knowing that you cannot broadcast it. At some point before block 800,000, if I'm still alive, I can either simply open my safe, destroy the transaction, and replace it with a new one which cannot be broadcast until block 900,000, or (if you already have the signed transaction), I can move all my coins to a new address to invalidate your one, and then send you a new one timelocked to block 900,000.