Entry date on invitation letter?

I am looking to apply for an F,( 3-month, single entry) in the UK. My official invitation letter states my entry date is 7th Oct 2013 (not stating departure date), but I had to delay my entry date since I got this invitation, so now my entry date is 16th Oct. Is this letter still ok to use when applying for it? I don't think it will make much difference to the validity itself?