I have an issue with a vba code I developed - excel

Question

I developed the vba code below to calculate cumulative production up to one date in the column "K" by adding amounts in the column "I" corresponding to dates less than the one in the column "K"
Option Explicit
Sub Gp()
Dim date2 As Double 'Same p/z production Dates'
Dim Gp As Double 'Cumulative production volumes"
Dim fraction As Double
Dim Daysmonth As Double
Dim Days As Double
Dim i As Integer, j As Integer
Gp = 0
j = 3
For i = 4 To 43
date2 = Cells(i, "K")
Do While (Cells(j, "A") < date2)
Gp = Gp + Cells(j, "I")
j = j + 1
Loop
Daysmonth = Cells(j, "A") - Cells(j - 1, "A")
Days = date2 - Cells(j - 1, "A")
fraction = Cells(j, "I") * Days / Daysmonth
Gp = Gp + fraction
Cells(i, "M") = Gp
Gp = (Gp + Cells(j, "I")) - fraction
j = j + 1
Next i
End Sub
This code takes the first cell of the "K" column and look all the dates less than date in the "A" column. Then it add all the amounts in the "I" column for those corresponding dates. Then if the date in the column "A" is in the same month and year of the date in the column "K" it will calculate the fraction of the amount corresponding to those days. Everything is alright until 8/14/80 when calculating the fraction it gives -17 instead of 14 days for 8/14/80 - 8/31/80. It gives me 6304593 and it should give me 6350518.484.
This is a picture of my answers in the column "M" and the right answers in the column "N". They are very similar but not the same for some dates
Calculated cumulative and right answers next to it

Answer 1

"when calculating the fraction it gives -17 instead of 14 days for 8/14/80 - 8/31/80"
Not sure where you are getting the 14 days. Since 8/31/80 is in the future it would be 17 days after 8/14/80. Based upon your formula of 8/14/80 - 8/31/80, it arrives at -17 which is correct.
If you are okay to include dates greater than 'date2', you could use the built in 'Abs()' function to always make the result positive and that should correct the calculation.
Days = Abs(date2 - Cells(j - 1, "A"))

Answer 2

Output of 8/14/80 - 8/31/80 should be -17 only , instead of 14 days. Just to check, I have tried your code and replaced the value -17 with 14 then output i am getting is 7255151.33 instead of 6350518.484.
I think fraction should not be calculated in this case(8/14/80 - 8/31/80) as the primary condition is date in column K(8/14/80) should be greater than Date in column A(8/31/80) which is failing in this case.
And if it is required then you can check if the output of date2 - Cells(j - 1, "A") is negative then replace it with Cells(j - 1, "A")-date2 to make the result positive.