There are userform combo-boxes, activeX combo-boxes and content control combo-boxes.
The following should point the way. I have added message boxes at each stage. Frankly I wouldn't use combo boxes for dates unless the dates are fixed in code as there is too much scope for error. Use a VBA date picker instead.
Dim iDiff As Single, iCalc As Single
Dim DateStart As Date, DateEnd As Date
Dim iVal As Single, iPercent As Single
DateStart = CDate(Me.ComboBox1.Value)
DateEnd = CDate(Me.ComboBox2.Value)
iVal = Me.ComboBox3.Value
'Remove % from the percentage
iPercent = Replace(Me.ComboBox4.Value, "%", "")
MsgBox iPercent
'Convert percentage to decimal
iPercent = 1 * iPercent / 100
MsgBox iPercent
'Calculate date difference
iDiff = DateDiff("d", DateStart, DateEnd)
MsgBox iDiff
'get final value and format to 2 decimal places
iCalc = Format(((iVal * iPercent) / 365) * iDiff, "0.00")
MsgBox iCalc