You can't if you use windows excel standard date system,
you can if you use Mac's 1904 system under tools>calculations
note that you would add 1462 days to any date you previosuly had
so you would need to put 1462 in a cell, copy it and select all dates that
were converted and do edit>paste special and check subtract
There are other ways like using conditional formatting and red fonts
=IF(cell1-cell2<0,cell2-cell1,cell1-cell2)
then use format>conditional formatting , formula is and
=cell1-cell2<0
then select format and red fonts
Of course if you are doing calculations it might be a bit tricky if there
are many
negative time values.
Regards,
Peo Sjoblom