THIS! Not having a defined Date format has caused so much confusion with consumers of our API since every json API might have a different ISO date format (or some homegrown madness).
1. ISO UTC 2. ISO local 3. Epoch 4. 2018-04-20 5. Jan 01 2016 11:58am
Probably others that I can't recall. It's a damn mess.
I never understood why simple integer unix timestamps are not more prevalent in API-s? Or some monotonic count from any reasonable epoch, depending on the context. How many APIs really have to ever return dates predating unix?
That will probably cause a twitch to any dev that used Asp.NET up until a few years ago.