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I am adding codes in order-detail.tpl to show a estimated date of delivery to customers from that is not available any module compatible with PS 8

My variable of order date is: {$order.details.order_date|date_format:"%A, %e %B of %Y"}

I want to join together {$order.details.order_date} with the code {"+8 days"|strtotime|date_format:"%A, %e %B of %Y"}

My full code of smarty now is: 

 {assign var="Delivery_day" value=$smarty.now|date_format:"%u"}
  <b>{l s='Estimated delivery day'}</b>
    {if $Delivery_day == 1}
        {"+8 days"|strtotime|date_format:"%A, %e %B del %Y"}
    {/if}
    {if $Delivery_day == 6}
        {"+9 days"|strtotime|date_format:"%A, %e %B of %Y"}
    {/if}
    {if $Delivery_day > 0 && $Delivery_day < 6}
        {"+8 days"|strtotime|date_format:"%A, %e %B of %Y"}
    {/if}

I tried with {"$order.details.order_date+8 days"|strtotime|date_format:"%A, %e de %B del %Y"} but it give me error, i am not expert.

Can you help me?