I believe the aperture is cut before applying the enamel. The outcome of the enamelling process is sure to be a little bit hard to predict and the painted frame has to be aligned to the 3 o'clock marker.
I couldn't imagine trying to cut into a cured enamel surface. Seems like it would shatter or at least crack. Besides at a nominal viewing angle, I bet it visually centers the date in the frame better!