How can I change a Datetime field to Date field based on a boolean?

is_half_day = fields.Boolean(string="Is half day?")
date_from = fields.Datetime('Start Date', 
                            readonly=True, 
                            index=True, 
                            copy=False,
                            states={'draft': [('readonly', False)], 'confirm': [('readonly', False)]}, 
                            track_visibility='onchange')
date_to = fields.Datetime('End Date', 
                            readonly=True, 
                            copy=False, 
                            states={'draft': [('readonly', False)], 'confirm': [('readonly', False)]}, 
                            track_visibility='onchange')