class ActiveSupport::Duration

def +(other)

are treated as seconds.
Adds another Duration or a Numeric to this Duration. Numeric values 
def +(other)
  if Duration === other
    parts = @parts.merge(other._parts) do |_key, value, other_value|
      value + other_value
    end
    Duration.new(value + other.value, parts, @variable || other.variable?)
  else
    seconds = @parts.fetch(:seconds, 0) + other
    Duration.new(value + other, @parts.merge(seconds: seconds), @variable)
  end
end