Increase decrease time with a spin button

I don't know what spInviteAtt.SmallChange is, but you wouldn't put it inside Now. Now is literally a string of text.

When you used CDate, you were casting spInviteAtt.SmallChange as a Date/Time variable, regardless of what variable type it actually is. The same casting works for time. So this code should work:

Private Sub UserForm_Initialize()
Reg17.Text = Format(Now, "h:mm:ss")
End Sub

Private Sub spInviteAtt_SpinDown()
Reg17.Text = Format(CDate(Reg17.Text) - CDate(spInviteAtt.SmallChange), "h:mm:ss")
End Sub

Private Sub spInviteAtt_SpinUp()
Reg17.Text = Format(CDate(Reg17.Text) + CDate(spInviteAtt.SmallChange), "h:mm:ss")
End Sub