Do Loop Until problem (Won't Stop)

Hi
I have a problem

Question 1

My loop won't stoppp

Dim startTime As Date
Dim endTime As Date
startTime = "9:30:00"
endTime = "18:00:00"

Do
blah
blah
startTime = startTime + "0:10:00"
Loop Until (startTime = endTime)

It won't stop?

Question 2

minute2 = DatePart("n", startTime)

If minute2 = 20 Or 50 Then
blah
blah
else
blah
blah
End If

The above works if the if statement is only looking for 20 . After I put the Or in it it  just keep staying on the top and won't go to else.
Any idea why?

Thanks

Please help