Hi Rodrigo,

Since you have dates in your zoombar I'll assume that you are using a DateTimeAxis in the XamTimeline.  DateTimeAxis has properties called VisibleMinimum and VisibleMaximum which are updated when the zoombar thumb is resized or moved around to reflect what the date range is on screen in the timeline.  You can use these properties to get your date range.  I.e. you can handle the XamTimeline.Zoombar.ZoomChanged event and inside the event access the XamTimeline.Axis, cast it to a DateTimeAxis and then inspect the VisibleMinimum and VisibleMaximum properties.

private void Zoombar_ZoomChanged(object sender, ZoomChangedEventArgs e)
{
    var axis = timeline.Axis as DateTimeAxis;
    var minDate = axis.VisibleMinimum;
    var maxDate = axis.VisibleMaximum;
}