[edburdo]

I'm not in your shoes, so this is all theory.

I'm with you in that I'd prefer to stay to see it into production. However, you have a commitment to your employer in that you agreed to start on Date X.

So, I see two options.

1) Explain to your new employer why you think you should delay starting. Go with what they say.

2) Explain to your current employer that the dates are already set, and that you need to leave.