From the Date::Manip documentation:
%Xh : print the value of field X and all
larger units in terms of X
so you are asking for the values of the fields of d (day) and higher in terms of days.
The values of all fields higher are 0:0:0:0, which is a total of 0 days, so what you got is expected.
It appears to me that what you are really asking for is the value of the delta in days, but ignoring the fact that a day is not strictly 24 hours (due to daylight saving time).
The following will do that:
Delta_Format($delta,'semi',0,'%dt')
=> 4
