If you have the ground terminal of your device hooked up to the ground on an oscilloscope the first part of the wave will show a positive voltage at (Time 1). When the ground on your device switches to positive that will be your reference ground on your scope (Time 2). The ground now going to the positive on your scope will now appear negative in reference to the possitive voltage on the ground of the oscilloscope. The second part of the wave will be negative.