Function not working because I use dates? - vba

Question

I’m trying to write a function that depends on dates, but I can’t get it to work. Excel keeps telling me that a value that is used in this formula have the wrong data type. So it has to be the date, because I have another function that is exactly the same except for the add of a datevalue.
Someone writes a date in the Excel-cell C10 and then I wan’t the function to summarize certain numbers (predicted inflation rates, column P) from a table (on another sheet in the same workbook) in which it depends on begin-dates (column I) and end-dates (column J).
I’ve checked that Excel interprets cell C10 as a date, it does but with a serial number.
But I don’t see why that is a problem since I use it to compare with other dates (serial numbers)?
Public Function FetchFactorInflation(Product As String, ArgName As String) As Double
Dim Datum As Date
Datum = DateValue(Worksheets("InputTariff").Range("C10").Value)
Dim MinDatum, MaxDatum As Double
MinDatum = "<=" & Datum
MaxDatum = ">=" & Datum
FetchFactorInflation = Application.SumIfs(Worksheets("tariff").Range("P:P"), _
Worksheets("tariff").Range("I:I"), MinDatum, _
Worksheets("tariff").Range("J:J"), MaxDatum, _
Worksheets("tariff").Range("K:K"), Product, _
Worksheets("tariff").Range("L:L"), ArgName)
End Function

Answer 1

The wrong data type comes from here:
MaxDatum = ">=" & Datum
MaxDatum is a double, and you are giving >= to it. It cannot be converted to a Double. Try a way around.
The quickest solution would be to write like this:
Dim MinDatum, MaxDatum
instead of
Dim MinDatum, MaxDatum As Double
Thus MaxDatum would be easily converted to what you need, because it would be variant as well as MinDatum.