按时完成任务用英语可以说作 "complete tasks on time"。
来源："complete" 意思是完成，"on time" 意思是按时。
例句：
It is important to complete tasks on time in order to meet deadpnes. (重要的是按时完成任务以达到最后期限)
I always try to complete my tasks on time so that I don't fall behind. (我总是尽量按时完成我的任务，以免落后)
She is known for her abipty to complete tasks on time, even under pressure. (她以能在压力下按时完成任务而著称)