time bomb 
noun
1A bomb designed to explode at a preset time.Phil reappeared just below Leif as the time bomb exploded....
- The time bomb exploded, the sound resonating in a disconcerting fashion off the interior walls.
- What need for creating a fuse for a time bomb could Price possibly have while stuck in a POW camp?
1.1A developing and problematic situation which will eventually become dangerous if not addressed: the demographic time bomb...
- But where such treatment is painful or debilitating, many would have to gamble their current health and lifestyles against the risk that their time bomb would eventually go off.
- China will be rid of a dangerous time bomb in its backyard.
- The situation is like a time bomb waiting to detonate.