I thought this was a personal project. Not trying to be a dick, just offering helpful advice. The reason to use the ISO 8601 format is to avoid ambiguity, you should always provide the timezone when providing a timestamp. The Unix timestamp (milliseconds from epoch) is unambiguous. And if your program was to read that, you get the correct time without question. The time and date however, without providing a timestamp, a programmer using that data could make a wrong assumption that the timezone was their local timezone.

This isn't a JSON issue, it's just generally programming advice, to avoid ambiguity whenever possible, in order to avoid bugs. Just trying to be helpful. I have over 10 years experience as a professional software developer at a senior level.

Time and date handling is incredibly difficult and one of the greatest sources of bugs in computer programming. Between localization, different applications of daylight savings time, leap years, etc. there are an amazingly large number of ways to make mistakes when dealing with time and date. Always use libraries with dates, never try to roll your own date/time code. You will almost without fail create some sort of bug.