Ramses the slippage message I believe happens more since Dave introduced the end time as an approx end rather than the end of the scenario (I think anyway). If thats the case and the slippage causes actual problems with reassessments maybe it might be best to revert back to the end time being the end of the scenario? The start and end times are something I never actually use only to increase arty bombardment times..it was a new feature for BFTB, one that I never really got around to using as I found the slippage and crib message coming up often and it all got abit confusing..now because I think the end time not being he end of the scenario the slippage messages happen anyway.
If I watch the game feedback correctly (and accurately recall what Dave said about implementing the time constraints for movement), the times that are truncated from "end game" are moves as constrained by an assigned pace (normal / fastest) and preferred routing type (avoidance, covered, quickest, etc.).
The various forms of direct defend, probe and attack orders default to the end of the game.
I incur slippages when setting an end time for the task and not fully taking into account distances, or unit quality / training / health status for their potential effect on response time (e.g. they stop at night to take their normal rest because I forgot to specify they could take minimal or no rest at all).
In all, the slippages are understandable within the bounds of what a commander would face on a battlefield encumbered with Fog of War, troops being ordered to do more than should be expected, and distances that need to be traversed to reach the point where the order is implemented.
Hope this helps.