If I want to write a date in the Gregorian calendar, I can write it in the ISO 8601 format; today would be 2016-02-20. The advantage is that there are many parsers available to let computers read in this kind of date. But what if I want to write a Julian calendar date, as astronomers often do? Is there a similar widely-accepted format, and selection of parsers, to use with the Julian calendar.
While I'm at it, what if it's an ancient date where we don't know how many solar days ago it was; instead it is an astronomical phenomenon calculated in Terrestrial Time (more or less atomic time) or by dating decay of radioactive elements? Is there a standard way of writing such a date?