Textbox Time Calculation

I'm attempting to calculate the total number of hours between a start time and end time using textboxes on a Userform.

Here is my latest attempt:

Private Sub D24_Exit(ByVal Cancel As MSForms.ReturnBoolean)
Dim myvalue As String
Dim myhour As String
Dim myminutes As String

myvalue = D24.Value

Select Case Len(D24)
Case 1
myhour = "0" & Mid(myvalue, 1, 1)
myminutes = "00"
Case 2
myhour = Mid(myvalue, 1, 2)
myminutes = "00"
Case 3

The above codes are an attempt to force an entry as time (7, 07, 700, 0700=7:00 / 13, 1300=13:00 etc.). However, the result is not correct.

For example:
Start Time in Textbox(D24)=07:00
End Time in Textbox(D25)=17:30
Result in Textbox(D27)=0.4375
Desired Result in Textbox(D27)=10.5

I've never calculated times in textboxes,