on a clock 赶时间=somebody is running out of time to do things，口语中使用。如果你碰到he is on a clock ，那不是催促，而是指时日不多。
扩展beat the clock 赶时间（做某事），提前完成任务。
【例句】
The work went well and we beat the clock, finishing nearly two hours before the deadline.
工作进行得很顺利，结果我们提前完成了任务，比最后限期前了约两小时。
After dawdling for years, governments must race to beat the clock.
虚度多年光阴后，各国政府现在必须争分夺秒。
