Whould it be UNIX timestamp format or some ISO?
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Considering that API consumer are more programming languages. (Java, C++, JavaScript)
UPDATED2
And what about this problem? http://beyondrelational.com/blogs/nakul/archive/2011/02/10/unix-timestamp-and-the-year-2038-problem.aspx And what about dates before 1970?
Some ISO standard. It's a standard for a reason.