JSON itself does not specify how dates should be represented, but JavaScript does.
You should use the format emitted by Date's toJSON method:
2012-04-23T18:25:43.511Z
By Marjaana Jokinen on February 20, 2019: This is the story of my learning journey teaching myself how to use xAPI. I share resources and xAPI examples that helped me get started.
Learn more about how the same JSON document can be parsed with different values across microservices, leading to a variety of potential security risks.