You don't need to use SiteMap nodes for this. If you page inherits from mojoBasePage, you can do this:
if(CurrentPage != null)
{
lastModified.Text = CurrentPage.LastModifiedUtc.ToString();
}
If you're not using mojoBasePage, you can get current page with:
using mojoPortal.Business.WebHelpers;
PageSettings currentPage = CacheHelper.GetCurrentPage();
Hope it helps,
Joe