Tian-Po's Water Clock

{t}: Add {c}.
{1}, {t}: Tap target creature. That creature doesn't
untap during its controller's next untap step.
{c}, {t}: Tap target creature. That creature doesn't
untap during its untap step as long as Tian-Po's
Water Clock is tapped.
At the beginning of your untap step, you may
choose not to untap Tian-Po's Water Clock.