Is there a variation of ISO 8601 for julian calendar dates? 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?
Astronomers use "Julian day numbers" to indicate time. The Julian day number is the number of days since Noon GMT on January 1st 4713BC (in the Julian calendar). A time is then a single real number. For example, 00:30:00.0 UT January 1, 2013 (Gregorian), is 2456293.520833 (wikipedia)

So except to specify the epoch, the Julian calendar is not in use by astronomers to specify dates. There is no international standard equivalent to iso8601 for representing dates in the Julian calendar.

Ancient dates are often expressed as years before present, where "present" is 1950. 1950 is chosen as it preceeds most of the atmospheric nuclear testing that dumped massive amounts of C14 into the atmosphere and made carbon dating only valid for years before 1950.