You could use Email Scripting. Information: http://developers.marketo.com/documentation/email-scripting/
You could potential just code it in the template. But the usuage is limited to the email only, and not to the entire program.
Another workaround is to create a new field in field management, with Next Webinar Date. And run a recurring batch to update the value every week. Instead of using {{my.nextwebinardate)) you would be using {{lead.nextwebinardate}}. Not elegant though.