Here are some useful ways to achieve it:
datetime = DateTime.now => Sun, 26 Oct 2014 21:00:00
datetime.today? # => true
datetime.to_date.past? # => false (only based on date)
datetime.to_date.future? # => false (only based on date)
datetime.to_date == Date.tomorrow # => false
datetime.to_date == Date.yesterday # => false