How do I calculate the length of our marriage?

There are two different dates to pay attention to when you calculate the length of your marriage.   

Are you trying to calculate the length of marriage for social security benefits or spousal military benefits?
For federal benefits, the date of marriage is measured from the date of marriage to the date of termination of status (the date your divorce decree is finalized).

Are you trying to calculate the length of marriage for spousal support and separate property?
California uses the date of separation as the date for the end of marriage to measure spousal support and the date on which all earnings become separate property.