If the calendars vary across the list of cubes then `convert_units()` will raise an error:

```python
>>> cube.coord('time').units
Unit('hours since 1970-01-01 00:00:00', calendar='360_day')
>>> new_unit = iris.unit.Unit('hours since 1970-01-01 00:00:00', calendar='gregorian')
>>> cube.coord('time').convert_units(new_unit)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../coords.py", line 647, in convert_units
    self.points = self.units.convert(self.points, unit)
  File "/.../unit.py", line 1855, in convert
    (self, other))
ValueError: Unable to convert from 'hours since 1970-01-01 00:00:00' to 'hours since 1970-01-01 00:00:00'.
```