I personally love this question, since I've thought about it myself! The gradually evolving personality of Homura throughout the story(who becomes more stoic and condescending at each timeline) complements the strangeness of the time manipulating "shield".
TL; DR version
Homura can use her magical shield to:
- reverse time (possibly constrained to certain rules)
- stop time (for a limited amount of time)
- store/retrieve anything (possibly in n-dimensional space)
- manipulate objects in space
- protect herself from anticipated attacks
- create a shock-wave of air around her
- make a fashion statement (I mean, just look at it, so fabulous!)
Homura's shield is undoubtedly the most mysterious "weapon" of all magical girls. The shield allows Homura to travel through time repeatedly to relive her first meeting with Madoka. The nature of temporal manipulation or it's range have not been clearly explained in the anime/Rebellion movie. However, it seems that it can be used only after a month of time has passed(as described in @homura 's answer, derived from Urobuchi's interview).
Homura's uses her "shield" in multiple situations to stop time. This certainly implies that the whole universe is at a stand-still(without which a lot of anomalies would surface at possible boundaries of time manipulation).
"It's powerful" would be an understatement for such a device. Notice the airplane in the background!
A detailed observation of the instances of Homura's usage of the shield would prove that it can do much more than just time manipulation(either reversing or stopping). Homura uses it as an (infinitely large?) storage space and brings out armaments when necessary.
Further, during her fight against the Walpurgisnacht, Homura controls and uses a large truck in combat, which was enveloped in a purple gleam. As time and space are connected, I think that this scene suggests space-time manipulation.
Another similar space manipulation scene occurs when Homura blocks off Walpurgisnacht's flames with her shield. She protects herself by deflecting the flames in a defensive move.
Also, in episode 1 of Madoka Magica(at 19 minutes 8 seconds), Homura clears off the fire extinguisher fumes by a blast of air using her shield...(maybe she set off an explosive charge to disperse the fumes, but this seems unlikely since she stays in the same spot throughout the scene)
I really love the
Madoka Magica series. I hope I've covered all the unique instances of Homura using her shield!