Maybe each time agency has control over certain types of events depending on what's changed. Or each have jusistiction over certain times. My guess would be that if someone screws with the timeline then the time agency closest to it in the future is the one who responds, if they fail then the next one ahead has all the time in the world to eventually step in and take control, etc etc slowly going further and further into the future until eventually one of them does something.
EDIT: Similar to law enforcement today maybe. Local sheriff is in charge but if he doesnt handle it right or quickly enough the local police take over, if he fails then the FBI step in and take control,etc, and up the ladder it goes except this ladder is a temporal one, the future into the future obviously the more knowledge and control you have.
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