Modifying Child Support in California - 18 Years Old

My question involves child support in the State of: California

I actually have a 2 part question:

My oldest daughter turned 18 on Nov. 1, 2008. I have not had the child support modified yet. I had been trying to come to an agreement with my ex without going to court. But that isn't working. Will the judge go back to the date she turned 18? Thus giving me credit for the extra I have paid.

My ex is now saying that he is going to ask the judge to give him more support for the period of July 2007 until Nov. 2008. We have a custody agreement for 50/50 custody, however our oldest daughter lived exclusively with her father during that period. He never asked for a change during that period, but now that we can't come to an agreement he is saying he will ask the judge to modify the amount to reflect full custody to him for that child during that period and to give him extra money. Can you do this in California?

Does the original support order specify an end date for CS?

It says age 18.

If the support was supposed to end when the child turned 18 then why did you keep paying? I'm confused :confused:
You didn't/don't need to modify anything - you should have just stopped making payments.
And you and your X were both in violation of the court order by not having the child 50/50. Someone else here will have to let you know if he can ask for a retroactive change in the support amount.