Errors caused by times not convertible to datetime

[stephenworsley]

🐛 Bug Report

Time coordinates can sometimes have units which will cause an error when you try and convert thier points to datetimes. For example, when the units are in "years since date" format. This causes an error when plotting against this time axis or when printing a cube with a scalar coordinate of this form. In such cases, it might be appropriate to represent the raw points (and raising a warning) rather than failing outright.

This could potentially by solved by adding try/except in places like here:

iris/lib/iris/coords.py

Lines 2166 to 2169 in b2b4e3b

	if self.units.is_time_reference():
	point = self.units.num2date(point)
	if bound is not None:
	bound = self.units.num2date(bound)

[acchamber]

bug_ncdump.txt

ncdump of file in question that gives this error

[rcomer]

Think SciTools/cf-units#279 may be relevant here.