Or take the desk where the light behaves differently:
Picture this screen lying flat on your desk. Its top edge points north. On an upright monitor, that is the direction you face while looking at it. The sundial turns itself to true north.
Sun reaches the desk through a window with small panes. Choose which side of the screen it is on, or take the walls away.
← → move ten minutes (with Shift, an hour). ↑ ↓ move a day (with Shift, a week). Space plays the day, Y plays the year, C turns the card, N returns to now. Click the card to turn it too.