Rails convert DateTime to date in 'where' query

I need to make a query to database and filter Event by start date, but column type is DateTime. I made scope in model:

scope :day, -> (start_date) {where start_date: start_date}

It works fine for same DateTime value, but I need a filter to get Event only by date, not DateTime.

I have PG database and try:

  scope :day, -> (start_date) {where("start_date:date =?", "#{start_date.to_date}")}  

but I get an error