The weapon that looks like Homura's shield is actually a sand timer. When the flow of the sand is blocked, time is stopped. And when there is no sand on the upper part of the sand timer and then the timer is reverted, one month's worth of time is turned back. But before that stage is reached, only time stopping is possible. This means the special power of Homura is the ability to manipulate one month's time's worth of sand in the sand timer. As for the time passed and then turned back, since it was not the focus in this work I did not think it through very thoroughly. I think you can say that the world split up and became parallel worlds there.
(NewType 2011-05, interview with Gen Urobuchi)