Contingent contract timing: enforceability depends on whether the specified uncertain event occurs within the fixed time or becomes impossible. Contracts contingent on a specified uncertain event within a fixed time depend on that event as a condition precedent: obligations to do or not to do something if the event happens within the fixed time become void if the event does not occur by expiry or becomes impossible beforehand; obligations contingent on the event not happening become enforceable when the time expires without the event, or earlier if it becomes certain the event will not occur.