H1B client letter should be backdated or current dated with a future project and for how long should the project be?

I applied for H1B visa this year through a small consulting firm and picked up in the lottery. No client letter was submitted at the time of application in april, as I live in India and there is no client at that time. Now, received an RFE requesting for client letter.If I acquire a client letter now, does it have to be back dated to March / April 2017 or does the client letter have to be with the current date for a future project and for how long?