回複：Thanks, but still not quite clear.

If you start to work for B from 4/1, about one month after the approval, it is still ok to say you are employeed by A between till 4/1. http://www.immihelp.com/visas/h1b/h1-visa-transfer-faq.html

It's not completely up to you to determine when you started employment at B company. You have I9 and pay record that establish your employment.