<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _field_sort_name(cls, name):
 """Get a sort key for a field name that determines the order fields should be written in. Fields names are kept unchanged, unless they are instances of :class:`DateItemField`, in which case `year`, `month`, and `day` are replaced by `date0`, `date1`, and `date2`, respectively, to make them appear in that order. """

        if isinstance(cls.__dict__[name], DateItemField):
            name = re.sub('year',  'date0', name)
            name = re.sub('month', 'date1', name)
            name = re.sub('day',   'date2', name)
        return name