The easiest way would be to update the countdown on each block (a drawback would be that the time to forge will jump around constantly).
But since you need to send your secretPhrase along with the request for the deadline, making that automatic (without the users expressed consent) has the risk of a user on a public node sending his passphrase once. Then realizing he/she doesn't want/trust that and the client will continue doing so.
This is a first basic implementation of the forger countdown, I can imagine a next release to show the current behavior only when you are on a public node.
Honestly, I understand why do you display the countdown, but from experience with Nxt I think it just confuses newbees.
How about instead of calling getForging you just make the calculations on the client side.
Instead of a countdown you could just display the avarage forging time based on balance, coin supply, and last base target. It could be updated with every block.
"You will forge a block an avarage of x weeks/days/hours", is good information, and less confusing than the countdown that jumps around and in no way accurate.
Or if you love the countdown you could check last forged block time, make the above calculation in client, and create a countdown. This way there is no need for passhrase.