Had 2 cents I wanted to throw in. This could mean nothing, or could be seen in a few different ways.
Now first off, we may have an idea of just how difficult to near impossible it may be to try and look back in time. In Turn Coat, we have a Senior Council member trying to determine exactly what had happened in the murder of one of their own. Even through all his prowess, through all his attempts, and use of magic even he was a bit skeptical of, they where never able to verify exactly what had happened. Either such a thing was too difficult for him to manage, or it was simply completely outside of his realm of expertise. Note, even Harry had to do that himself the good old fashioned way, and it had to partially be confessed by one of the parties involved (note, I'm tired and it has been a while since I've read this particular book. I may not be remembering some of the finer details). Player characters can always be resourceful, they can always eventually out shine most of your NPCs. However, our Senior Council members, I would assume, should be considered as the absolute elite, the true authority of magic in most scenarios. If one of them couldn't do it, well...
Now, this could potentially be looked at another way. The Council member may have been trying to do everything he could, EXCEPT looking back in time. Through all the tools he had, he was unable to bring enough evidence to bear to verify exactly what had happened. If this is the case, than it may not be the idea that it is necessarily too difficult to look back in time, but that even this potentially minor act of chronomancy would be considered to be in violation of the 6th Law. If this was a gray area, the envolope may have been pushed. If it was not something considered to be breaking the Laws, than ALL avenues would have been pursued in attempts to verify exactly what had happened.
Now as I said, I'm tired, so I may be rambling a bit. But from my speculation at least, it could be assumed from that point in the books we may be able to assume we simply have an A) or B). That even this kind of divination or time magic is either very difficult to impossible to perform to a very definitive detail, or it is considered to be against the 6th Law, whether it would break it or not. My 2 cents to the conversation.