How to make a widget fill remaining space in a Column

Not 100% sure but I think you mean this. The Expanded widget expands to the space it can use. Althought I think it behaves a bit differently in a Row or Column.

new Column(
children: <Widget>[
    new Text('Title',
        style: new TextStyle(fontWeight: FontWeight.bold)
    ),
    new Expanded(
        child: new Text('Datetime',
             style: new TextStyle(color: Colors.grey)
        ),
    ),
],
)