I think Shanks's DF power is time related. To be specific. It's an Again Again fruit that could enable him to do so many things.
- He may have seen the events at marineford beforehand and went back in time to appear at the right moment.
- He may have used it to have enough time to master way too many skills in a short span of realtime. (3Hakis, Swordsmanship, Strategic Skills, etc.)
- It even supports the theory that Roger is Luffy and Shanks may be supporting Luffy's progress to keep everything in order until another event in time (that he may have triggered himself) occurs to transport Roger to Luffy's time or vice versa.