Dates returned in responses should be formatted like: yyyy-MM-dd'T'HH:mm:ss.SSS'Z'
I'm no datetime expert. Googling around, I could not verify that it is some sort of standard. Therefore, this seems like an arbitrary formatting decision.
Does this format have a proper name? If so, please document its name. This lets developers more easily find code to do the correct formatting.
If the format does not have a proper name/standard, please briefly justify the arbitrary choice (e.g., "This format string results in a high-resolution, human-readable date-time.")