SUBDATE, SUBTIME functions...

Yesterday I was little bit surprised when I found,
that these two function are totally diferent.
Whilst the first mentioned takes as the second parameter
the numeric value of days, which I want to subtract from my date,
the second takes a timeexpression - and returns the difference
between two times.
If think it is a lack of consequence. The difference between them
should be the date/time type of the first parameter, don't You think?
As the function SUBTIME  doesn't allow me to
subtract for example two hours from current time, could
someone suggest a workaround?
I would appreciate it very much :)
Best regards,
Marcin Pytel
Talex S.A.