class Dentaku::AST::Subtraction

def value(context = {})
  left_value = left.value(context)
  right_value = right.value(context)
  if left.type == :datetime || datetime?(left_value)
    Dentaku::DateArithmetic.new(left_value).sub(right_value)
  else
    calculate(left_value, right_value)
  end
end