The visa will be dated the day they grant it, irrespective of your departure date unless you request that they date it in advance.
You do not need a flight iterary to get the visa. I booked my flight after I got the visa because I got a really good deal on a ticket that was non-changeable and non-refundable. I knew I was going to get the visa, but I just didn't want any tiny problem to create a big problem, like if they needed more information or anything similar that would delay the visa process and cause me to possibly miss my flight and lose the $. I had all my paperwork in order, was obsessively organized with a binder and tabs and a table of contents, mailed my visa to New York and got it back the morning of day five after they received it.
So, if you have EVERYTHING in order and are super organized and send in your application now, I would think you would get it back soon and would be able to book a flight soon. However, if you can get a changeable ticket or feel lucky and like to gamble, get your ticket first.