“He who controls the past controls the future. He who controls the present, controls the past.”
There are many Earth timelines. The Shadow comes from one of those alternate timelines (or perhaps more than one). It is attacking our timeline by punching bubbles into our past that last no more than twenty-four hours. In each bubble, the Shadow is trying to change our history and cause a time Ripple.
That would be the end of our timeline and our existence. To achieve its goal, the Shadow attacks six points in time simultaneously; the same date, in different years. The Time Patrol’s job is to keep our timeline intact. The Time Patrol sends an agent back to each of those six dates to keep history the same.
Is today one of those dates?