VBA long overflow

r = A - Int(A / B) * B

VERSION 1.0 CLASS
BEGIN
  MultiUse = -1  'True
  Persistable = 0  'NotPersistable
  DataBindingBehavior = 0  'vbNone
  DataSourceBehavior  = 0  'vbNone
  MTSTransactionMode  = 0  'NotAnMTSObject
END
Attribute VB_Name = "Decimals"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = True
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = True
Attribute VB_Ext_KEY = "SavedWithClassBuilder6" ,"Yes"
Attribute VB_Ext_KEY = "Top_Level" ,"Yes"
Option Explicit

'local variable(s) to hold property value(s)
Private mvarDec As Variant 'local copy

Public Property Let Dec(ByVal vData As Variant)
'used when assigning a value to the property, on the left side of an assignment.
'Syntax: X.Dec = 5
    mvarDec = CDec(vData)
End Property

Public Property Get Dec() As Variant
Attribute Dec.VB_UserMemId = 0
'used when retrieving value of a property, on the right side of an assignment.
'Syntax: Debug.Print X.Dec
    Dec = CDec(mvarDec)
End Property

Dim dec1 As New Std.Decimals
Dim dec2 As New Std.Decimals
Dim dec3 As New Std.Decimals
Dim modulus As New Std.Decimals

Sub main()
    dec1 = "1000.000000001"
    dec2 = "1000.00000000000001"
    dec3 = dec1 + dec2
    Debug.Print dec1
    Debug.Print dec2
    Debug.Print dec3
    Debug.Print dec3 * dec3
    Debug.Print dec3 / 10
    Debug.Print dec3 / 100
    Debug.Print Sqr(dec3)
    modulus = dec1 - Int(dec1 / dec2) * dec2
    Debug.Print modulus
End Sub

1000.000000001
 1000.00000000000001
 2000.00000000100001
 4000000.000004000040000001
 200.000000000100001
 20.0000000000100001
 44.721359550007
 0.00000000099999
 1000.000000001
 1000.00000000000001
 2000.00000000100001
 4000000.000004000040000001
 200.000000000100001
 20.0000000000100001
 44.721359550007
 0.00000000099999

Function BigMultiply(ByVal s1 As String, ByVal s2 As String) As String

Dim x As Long
x = 7

Dim n1 As Long, n2 As Long, n As Long
n1 = Int(Len(s1) / x + 0.999999)
n2 = Int(Len(s2) / x + 0.999999)
n = n1 + n2

Dim i As Long, j As Long
ReDim za1(n1) As Double
i = Len(s1) Mod x
If i = 0 Then i = x
za1(1) = Left(s1, i)
i = i + 1
For j = 2 To n1
 za1(j) = Mid(s1, i, x)
 i = i + x
Next j

ReDim za2(n2) As Double
i = Len(s2) Mod x
If i = 0 Then i = x
za2(1) = Left(s2, i)
i = i + 1
For j = 2 To n2
 za2(j) = Mid(s2, i, x)
 i = i + x
Next j

ReDim z(n) As Double
Dim u1 As Long, u2 As Long
Dim e As String
e = String(x, "0")

For u1 = 1 To n1
 i = u1
 For u2 = 1 To n2
   i = i + 1
   z(i) = z(i) + za1(u1) * za2(u2)
 Next u2
Next u1

Dim s As String, y As Double, w As Double, m As Long
m = n * x
s = String(m, "0")
y = 10 ^ x
For i = n To 1 Step -1
 w = Int(z(i) / y)
 Mid(s, i * x - x + 1, x) = Format(z(i) - w * y, e)
 z(i - 1) = z(i - 1) + w
Next i
'truncate leading zeros
For i = 1 To m
 If Mid$(s, i, 1) <> "0" Then Exit For
Next i
If i > m Then
 BigMultiply = ""
Else
 BigMultiply = Mid$(s, i)
End If

End Function