In what format should a REST API return and accept timestamps? The two most popular ways are Unix time (or a slight variation thereof) or ISO-8601. Both have their strengths and weaknesses and both are equally popular as we shall see. A sample of 20 APIs yielded nearly a 50/50 split. Therefore, no matter if this holds any persuasion or not, one can walk away knowing that their method of choice given Unix time or ISO-8601, is common knowledge and shouldn’t present a steep learning curve to other developers.