The idea of having designated writing time in your designated writing space is awesome. It keeps you consistently writing, which should aid in your creativity and productivity. But let's be real: you can't plan for life.
Things happen. People may need your attention. You may have a long day which saps your energy. When this happens, kiss your writing time goodbye.
Lost time might be able to be reclaimed. If you end up shifting your designated writing time by an hour or two in either direction, you can still get your writing time in, even if it's not the exact same time as usual.
Keep in mind that this is a pendulum which can swing in either direction. You may be ready to close your eyes and fall asleep when inspiration strikes you. You have he best idea for finally getting your story idea off the ground and running. And not only that, you may actually have the energy to go out and write it for several hours. Make use of this valuable present! It may not be your designated writing time or your designated writing space, but when the spirit strikes you, roll with the punches.