I think you have to use the 1904 date system. Go Tools menu > Options...
command > Calculation tab > Select 1904 date system. But, if your workbook
has links to other workbooks that use the default 1900 date system, it could
cause erroneous results.
As a workaround, could you format the cell as a number, absolute value it (or
* -1), then use a special format to show "-" time ? ... or do you need to
calculate with it a bit more ?