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
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