I looked through the Addons on this site for death knights hoping to find something that would remind you to refresh Horn Of Winter when it's about to pop.  I noticed that Slice Commander (for rogues) has a very nice system that could easily be converted.  And a warrior shout mod could also be easily converted, since the two buff systems are similar.  If an Addon is already written and I overlooked it, I'd love for someone to point me in the right direction.

I use Smart Buff for that...Works like a charm.

zomg buffs also works

I wrote an addon that does this for me simply due to wanting low-overhead called "Horn of Winter Tracker" which is currently pending administrative review.  Be sure to search for it soon :)