How about this:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">

    <mx:Style>
        .dateChooserHeaderStyles {
            fontWeight: bold;
            textAlign: left;
        }
    </mx:Style>

    <mx:DateChooser headerStyleName="dateChooserHeaderStyles" />

</mx:Application>

Peter

Actually, I turned this into a full example. For more information, see “Aligning headers in a DateChooser control in Flex”.