All date/time values in Excel are positive.
As a result you will likely get "#######" in a cell formatted as a date/time with a negative value.
Try entering the time as a positive value and then in the cell that uses the value subtract it, i.e., subtract a positive value rather than adding a negative value.
I don't know what your formulas are, so I don't know if this suggestion is suitable. If you post more information with the formulas you want to use, maybe someone can suggest a better solution.