DatDiff for bonds

I do need to calculate interest rates on bonds. Though, in financial markets, a month has only 30 days and a year only 360 days. How to get the datdiff between the last payment date, say october 5 and today, dec. 2 with a month only having 30 days. The result has to be 57 days.
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How does this seem ?

Date1 = d1/m1/y1
Date2 = d2/m2/y2

' days in first unfinished month
part1 = DateDiff("d", Date1, 30/m1/y1)

' days in whole months
If m1 = m2 then
    part2 = 0
    part3 = 0
    part2 = (DateDiff("d", 01/(m1+1)/y1, 01/m2/y2) -1) * 30

    ' days in last unfinished month
    If d2 = 31 then
        part3 = 30
        part3 = DateDiff("d", 01/m2/y2, Date2) + 1
    End If

End If

Result = part1 + part2 + part3

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