from what date should we determine the value of marital assests in divorce, if we have been living separate for ten years but never filed for divorce?

Our only important asset is my employer sponsored pension plan which changed in value over the ten year period, so the value of it to divide depends on the date on which we consider the marriage over. And the related question is (since my income is bigger than my wife's at this point and I will have to pay some spousal support), can I be asked to pay spousal support for the period before the day in which we divide our assets, in which in theory we were married?