Thanks @bugsysha this is exactly what I needed. I still wonder how you could understand my desperate, succinct question :-)
For further readers :
I wanted to create a tailor-made resource (based on one of my app model called Training)
I ended up with :
in the tailormade resource nova\new_training.php :
/**
* The model the resource corresponds to.
*
* @var string
*/
public static $model = 'App\Training';
/**
* Build an "index" query for the given resource.
*
* @param \Laravel\Nova\Http\Requests\NovaRequest $request
* @param \Illuminate\Database\Eloquent\Builder $query
* @return \Illuminate\Database\Eloquent\Builder
*/
public static function indexQuery(NovaRequest $request, $query)
{
return $query->where('created_at', '>',Carbon::now()->subDays(1));
}