Calculate Pay For Shift Work With Different Rates Based On Shift Hours

a person works for certain hours and get paid according to the hours worked either by day or by night or a mix of both. Day payment is $8 when worked between 08:00 and 19:59 , night payment is $12 when worked between 20:00 and 07:59. The excel cell are formatted as datetime with yyyy-mm-dd hh:mm , the function works fine in getting the time information and checking whether the whole work is all day or all night , yet the if-then-else statements for calculation seems to be wrong!!

examples:

start = 2008-01-01 09:15 , end = 2008-01-01 11:40 , all day as it is between 08:00 and 20:00 and cost = 8/hr = 19.333

start = 2008-01-03 21:05 , end = 2008-01-04 02:05 , all night as it is between 20:00 and 08:00 and cost = 12/hr = 60.000

start = 2008-02-02 19:00 , end = 2008-02-02 20:05 , cost = 9.000 as 1 hour day = 8.000 plus 5minutes night = 1.000

Function prod(st As Date, en As Date) As Double
Dim shour As Integer
Dim smin As Integer
Dim ehour As Integer
Dim emin As Integer
Dim stod As String
Dim etod As String
pday = 8
pnight = 12
shour = Hour(st)
smin = Minute(st) + shour * 60
If (shour >= 8 & shour < 20) Then
stod = "day"
Else
stod = "night"
End If
ehour = Hour(en)
emin = Minute(en) + ehour * 60
If (ehour >= 8 & ehour < 20) Then.................