Help with H1-B transfer!!!THX!

I'm going to an interview and it is very promising. Just very confused by H1-B transfer process.Which date I can start to work for the new employer, and which date I have to start to work for the new employer (can't work for the previous one any more):

1.The date start for New: the date the attorney file the documents to INS, or the date he recieved the receipt back from the INS, or the date new H1-B approved?

2. The date can't work for the old any more: the date he recieved the receipt back from the INS, or the date new H1-B approved?


Please help, desperate!