How to center absolute div horizontally using CSS?

I’ve a div and want it to be centered horizontally – although I’m giving it margin:0 auto; it’s not centered…

.container {
    position: absolute;
    top: 15px;
    z-index: 2;
    width:40%;
    max-width: 960px;
    min-width: 600px;
    height: 60px;
    overflow: hidden;
    background: #fff; 
    margin:0 auto;
}