Returning dates/times as unix timestamps vs ISO strings

[GUI]

Feedback from our API Usability Testing: One of the participants preferred if all times being returned from our APIs would be returned as unix timestamp integers. I think most of our APIs typically return things times as ISO 8601 strings. The preference for unix timestamps is that they can be added and subtracted directly without having to first parse and create date objects in your language based on the ISO 8601 string.

[GUI]

And I guess for my own personal preference here, I prefer the readability of ISO 8601 strings instead of unix timestamps. The string approach does mean you have to parse it into a date object in whatever language you're dealing with, but I think the same usually goes for unix timestamps as well (it seems rare that I'm just needing to add or subtract dates as integers, but this obviously depends a lot of the use case). But I'd certainly like to hear anyone else's feedback or preferences on how they prefer to receive dates and times in an API.