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