Hi mary23,
A date of separation is simply determined by the date on which you intended to live separate and apart as husband and wife. This often results in two different dates of separation, since the intention of the parties are not always the same, but eventually the parties must agree on a valuation date before they may proceed with divorce.
A date of separation cannot be proved. If you were to obtain a divorce, the only evidence a court would ever need is a written oath by you or your husband stating that you and he separated on such and such a date.
If it was just an agrument you had and you do not view yourself as separated from your husband, then you are not separated. I hope things work out for you.
Lindsay