Get date from datetime fielld

hi ,
i want search events for a date selected (YYYY-MM-DD)  this is my query , in db's table i have datetime field:

Code:

     $lista_eventi_utente = $event_model ->where(SUBSTRING('start', 0, 5),$date)
                                         ->where('id_users',auth()->user()->id)
                                        ->findall();

but have error in  SUBSTRING , how can i get date from datetime?

Research the php functions to convert a string into datetime.