Make sure you mention that you want it to be post-dated (not extended), and provide them with the date you would like it to start (assuming it is no more than 3 months in the future), and you shouldn't have any problems. *
Congratulations and good luck.
*Not to make you nervous, but there was a very tense moment when I went to apply for my fiance visa ahead of time and was told by the interviewing officer that 'it's incase there is a death in the family, not for your convenience'. The officer eventually went back to his boss to check, and things worked out in the end. (To be fair, we think he got confused between having it post-dated, and having it extended).