(the) clock is ticking
1. There is only a finite amount of time left. The clock is ticking, so be sure to complete your exams efficiently so you don't have to skip questions. They have a chance to tie the game, but the clock is ticking. I know it is a pessimistic view, but in my mind, the clock is ticking on the human race.
2. Of a woman, there is a limited amount of time in which to be able to conceive a child. I've been very successful in my career and want to see it to its fullest, but I would also love to have kids, and I know my clock is ticking. For women who may want to have children, the clock is always ticking—a concern that men never have to worry about.