def convert_dates(self):
        '''
        This function converts the estimated "time_before_present" properties of all nodes
        to numerical dates stored in the "numdate" attribute. This date is further converted
        into a human readable date string in format %Y-%m-%d assuming the usual calendar.

        Returns
        -------
         None
            All manipulations are done in place on the tree

        '''
        from datetime import datetime, timedelta
        now = numeric_date()
        for node in self.tree.find_clades():
            years_bp = self.date2dist.to_years(node.time_before_present)
            if years_bp < 0 and self.real_dates:
                if not hasattr(node, "bad_branch") or node.bad_branch is False:
                    self.logger("ClockTree.convert_dates -- WARNING: The node is later than today, but it is not "
                        "marked as \"BAD\", which indicates the error in the "
                        "likelihood optimization.",4 , warn=True)
                else:
                    self.logger("ClockTree.convert_dates -- WARNING: node which is marked as \"BAD\" optimized "
                        "later than present day",4 , warn=True)

            node.numdate = now - years_bp

            # set the human-readable date
            year = np.floor(node.numdate)
            days = max(0,365.25 * (node.numdate - year)-1)
            try:  # datetime will only operate on dates after 1900
                n_date = datetime(year, 1, 1) + timedelta(days=days)
                node.date = datetime.strftime(n_date, "%Y-%m-%d")
            except:
                # this is the approximation not accounting for gap years etc
                n_date = datetime(1900, 1, 1) + timedelta(days=days)
                node.date = "%04d-%02d-%02d"%(year, n_date.month, n_date.day)
This function converts the estimated "time_before_present" properties of all nodes
        to numerical dates stored in the "numdate" attribute. This date is further converted
        into a human readable date string in format %Y-%m-%d assuming the usual calendar.

        Returns
        -------
         None
            All manipulations are done in place on the tree