I'm not sure it's a bug or not, suppose cell is Cell object. cell.is_date will invoke self.style.number_format.is_date_format
in function is_date_format, the code is:
return any([x in format for x in self.DATE_INDICATORS])
self.DATE_INDICATORS is 'dmyhs'
in office, when cell is numeric, negative setting could have color, then the format_code may be: 0.00_);[Red]\(0.00\), it has `d`
i find this when use cell.value, it raise a ValueError.