document.getElementById(self.prefixA + cam_name + i).addEventListener('dragstart', function(event) {self.posX = event.clientX;});
document.getElementById(self.prefixA + cam_name + i).addEventListener('dragend', function(event) {self.posX_fin = event.clientX;
                                                                                                  self.scroll_frise(this.name, event);});

this.scroll_frise = function(pixdate, e)
    {
        alert(this.posX); // give me a correct number ex 147
        alert(this.posX_fin); // give me an incorrect number like 1799
        if (this.posX_fin != this.posX) {
            this.posX_fin = this.posX_fin - this.posX;
            pixdate = pixdate + this.posX_fin * this.myunit;
            this.mydate = new Date(pixdate * 1000);
            this.refresh(this.updatefrise);
        }
    }