You could use the where field and SQL's built in DateTime function:
Last 7 days:
WHERE ArticleDate >= DATEADD(day, -7, GETDATE())
For a macro you would be able to use:
{% (DateTime.Now - DateTime.Now.AddDays(-7)) |(identity)GlobalAdministrator%}
Edit: I just re-read the question and realized you are trying to find the age of content:
(DateTime.Now - ArticleDate).TotalDays