Time '0:00' is not saved correctly after loading and saving a file

Created originally on Bitbucket by rgeens (Raf Geens)

Midnight seems to produce a strange result when loaded and saved through openpyxl. Other times did not seem affected.

Steps to reproduce:

1. In Excel, format a cell as Time with an English (US) locale. Put a 0 value in it.
2. It will display as '0:00' and the cell contents will show as '00:00:00'.
3. Format another cell as Custom with the 'hh:mm' format. Put a 0 value in it.
4. It will display as '00:00' and the cell contents will show as '00:00:00'.
5. Save the file as time_example.xlsx (see attachment)
6. Load and save the file in openpyxl:

import openpyxl
wb = openpyxl.load_workbook('time_example.xlsx')
wb.save('test.xlsx')

• 7a. Open test.xlsx in Excel. Both cells will display as hash signs and the contents will be -1.
• 7b. Open test.xlsx in LibreOffice. Both cells will display as normal (besides the custom field showing as 0:00 instead of 00:00), but the contents will be '-24:00:00'.
• 7c. Open test.xlsx in Google Spreadsheets. Both cells will display as normal (besides the custom field showing as 0:00 instead of 00:00), but the contents will be '29/12/1899 00:00:00'

Versions:

• openpyxl: 2.5.3
• Python: 3.5.5
• Excel: 2016, Version 1803
• LibreOffice: 5.4.6.2

Attachments: excelgooglesheetslibreofficetime_example.xlsx