hi i want to addition in date in odoo directives in odoov8

in reports i print invoice date using

<span t-field="o.date_invoice"/>;

now i want to add days in this date meansmy invoice date is 10/01/2015 and my pament term days is 15 so i want to add 15 days in date in report.xml file through qweb directives so output is 10/16/2015 (10/01/2015(mm/dd/yyyy) + 15days = 10/16/2015)

i get payment term days from this

<tr t-foreach="o.payment_term.line_ids" t-as="pl">

<td> <span t-field="pl.days"/> </td> 

</tr>

so how can i do addition in date in qweb directives