format [hh:mm] and negative values

[K0ertis]

Greetings,

when I use the format [hh]:[mm] with the number 7:42 for example the cell look like: [hh]:42.
Also when I subtract two times for example 7:00-8:00 it shows only interrogation marks ??-???-?? ??:??:?? as shown in number example table: http://sternlife.stern.nyu.edu/m/page/help-en/number_format_examples

Is there a possibility to handle negative numbers too?

Thanks