When I can start working when applying for H1B?

I’m currently on a valid L1B visa. Let’s say I find a new employer that want to hire me tomorrow.

Can the new employer file for H1B visa now or he has to wait untill April? If the new employer does NOT have to wait till April, when I can start working for that new employer, right away after he files the application?

  1. Wait till April to file and October to start working

  2. N/A