With the right key you can unlock the sun to yesterday, or a previous day. The sun is in charge of the day changes, so if you use a key to unlock it, you can control the flow of time and return to yesterday, or a previous day in the past. Behold the power of the sun.
Time traveling to a previous time, like yesterday, can be rough on the body. As such, you wear a small sack (さく) to hold your valuables while you’re ripped through the sands of time. The sack keeps your important stuff from getting lost. The sack holds your key to the sun as well.