"When can you start?"
In other words, your new employer needs you right now, if not yesterday, and the project is going to go off the rails if they don't get some skilled professionals in right away.
So what do you do? You rearrange your schedule. You move around your appointments so that you can start 'post haste.' And you tell your new employer that your willing to start helping them out immediately.
And then what happens? They can't get a contract to you for two weeks becuase it has to 'go through legal', and 'finance wants to put off hiring until the end of the quarter next week', and 'they did a background check and they have a problem with all the criminal convictions on your police record.'
If you're a contractor, you've experienced it; that is, the pressing need to get you started, but the incessent delays in getting you what you need to start working on the project. And it's not just about getting the right documents signed. How often does a contractor get into an office where they actually have a desk ready for them? Or a computer? Or even better yet, a computer with the right software on it?
Onboard your new hires properly. For some direction in that vein, here's some onboarding advice: