Can I leave Thailand until the end of the day on my visa's expiry date, and how can I extend my stay if needed?

The visa expiry date indicates that you must pass through immigration by midnight of that date. If your visa ends on the 17th, you need to leave before that date's end. Extensions are possible for 30 days at a cost of 1900 Baht, or if you've had a prior extension, you might qualify for a 7-day extension. Otherwise, an overstay incurs a fine of 500 Baht per day, which could lead to immigration issues later.