Conversion to datetime.timedelta doesn't work when using read-only mode
Openpyxl's conversion of duration-looking dates to datetime.timedelta doesn't seem to work when using read-only mode. Here's a quick reproducer. Cell A1 contains '50:22:13'. ``` import openpyxl wb = openpyxl.load_workbook('test_duration.xlsx') ws = wb.active print(type(ws['A1'].value), ws['A1'].value) wb1 = openpyxl.load_workbook('test_duration.xlsx', read_only=True) ws1 = wb1.active print(type(ws1['A1'].value), ws1['A1'].value) ``` Result: ``` $ python3 test_duration.py <class 'datetime.timedelta'> 2 days, 2:22:13 <class 'datetime.datetime'> 1900-01-02 02:22:13 ```
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