Misc date fixes

[JohnBrinkman]

This issue summarizes some of the necessary code and documentation changes arising from discussion on https://github.com/adobe/json-formula/issues/117

• datedif() returns null when end_date < start_date. Instead, this should be a TypeError exception
• Remove the "md" unit option from datedif(). The documentation for the Excel version of datedif() states: "We don't recommend using the "MD" argument, as there are known limitations with it. The "MD" argument may result in a negative number, a zero, or an inaccurate result."
• Allow parameter overflow/underflow on time() function (just as we do for datetime()). e.g. specifying 62 minutes will result in 2 minutes and the hour value incremented.
• hour(), minute(), second() functions need to support negative values (dates before 1970)
• Added a millisecond method to now allow access to all the date parts
• Added tests for the weekday examples in the API docs
• Bad parameter to weekday now throws TypeError instead of return null
• For the toDate() functionality, document that we support only the RFC 3339 profile of ISO8601

[maxwyss]

Would it make sense if datedif() would return a negative number when end_date < start_date?

[JohnBrinkman]

Would it make sense if datedif() would return a negative number when end_date < start_date?

Maybe we could. But negative differences don't always make sense. For example, a common use of this function is to calculate someone's age. In that case, we never want a negative value, and the author will be better served by having the order of parameters enforced.

[JohnBrinkman]

Note as well:

• Excel enforces this same parameter ordering
• If you have two dates and you don't know which is earlier, you can order them with an expression such as: [d1,d2].datedif(min(@), max(@), "d")