[unlimited_power]

I believe you can use nLockTime[0] to construct and broadcast a transaction that cannot be accepted into a block at least until a particular time. Before that time is reached, you should be able to broadcast a conflicting transaction if so desired that is included in a block sooner, thus invalidating the previously broadcast transaction.

[0] https://en.bitcoin.it/wiki/NLockTime

[nwh]

That's right. You have to keep broadcasting it though, the transaction won't be retained in the nodes storage for a long period of time.