time bomb

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A bomb with a detonating mechanism that can be set for a particular time.
• noun Something that threatens to have an abruptly disastrous outcome in the future.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A bomb that has a mechanism such that detonation can be preset to a particular time.
• noun by extension A situation that threatens to have disastrous consequences at some future time.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a bomb that has a detonating mechanism that can be set to go off at a particular time
• noun a problematic situation that will eventually become dangerous if not addressed