Schedules are made early on with the best intentions, and no pressure, and retrospectively, to be out by a week over a 6 month project is actually pretty good. But nobody likes to slip.
Two things come to mind about schedule slippage, (both courtesy of Jim McCarthy):
- never trade a bad date for an equally bad date, and;
- a slip should be a net positive.
When you slip, there's serious pressure on the team to come up with a new date. Often you'll just pick a new date that's in the future - by virtue of it being the future, it's automatically better than the one you've just missed, which is now in the past. Giving into this impulse is dumb - it almost always leads to more slippage.
And slips happen because of things that you didn't know, which have since become known. So, that's not actually a bad thing. Better knowledge of how to ship the product can't be bad. What's bad is when you slip and you don't know why.
In our case, we have a better insight into what we need to do to hit the new milestone, we've revisited the specs and schedules, and we've set a realistic date that we can hit, so I think we're okay on both accounts.
Still not fun to slip though :(