Time Difference between two properties

Hello I need help with creating Time Difference property.

Property Values Examples:

Start Time: 02:57:37 PM
End Time: 02:58:01 PM

I have tried:

Option Explicit

CONST START_TIME_ID = 1213
CONST END_TIME_ID = 1214

dim StartDate_PV: set StartDate_PV = PropertyValues.SearchForProperty(START_TIME_ID)

dim EndTime_PV: set EndTime_PV = PropertyValues.SearchForProperty(END_TIME_ID)

if  ( NOT StartDate_PV.value.IsNull() ) AND ( NOT EndTime_PV.value.IsNull() ) THEN
   output = DateDiff( "d", StartDate_PV.value.value, EndTime_PV.value.value )
else

   output.SetValueToNULL MFDatatypeInteger

end if

But I'm getting Datestamp result: 12/30/1899 1:00 AM

Any solution to this?