Just to clarify for any newbies reading this post.
You do NOT decide what date the cow will calve (she will make that decision). You decide what date you will put the bull in with the cow (or start AI breeding) and what date you plan to pull the bull out (if your goal is to have a planned breeding program).
So, to play the game. If you put a bull in on 12-22 each and every year, and he bred the cow that day AND she decided to have it on 283 days, then the cow would have 82 days open (365 - 283)
That is easy math - if you have one cow.
As you increase the number of cows, and you plan on a 45 day or 60 day calving season, then the cows that calved at the end of the season won't have 82 days open, they may only have 22 days when breeding season starts.
As I said before, if you don't have a breeding program (set timed) then yes, it is very easy to have two calves within a 12 month period from the same cow. I have many cows that have produced 2 calves within 12 months - within my 60 day calving season. Not a big deal.
My goal is to maintain a healthy herd that makes my farm the most NET profit per cow.