Stanford’s Guidelines to Web Credibility:
Update your site’s content often (at least show it’s been reviewed recently).
People assign more credibility to sites that show they have been recently updated or reviewed.
Have you ever been to a website and you could just tell the content was old and stale? Maybe it wasn’t obvious right away, but as you browse through the site you begin to see things that perhaps don’t align with other things touted on the website. Or maybe you see a “Valentine’s Day Special” still hanging around a week AFTER Valentine’s day.
Small thinks like this can really turn visitors off. One the less obvious stuff you have a bit more leeway–until someone actually recognizes the contradictions–but on the obvious stuff, it can be a clear sign that you’re not paying attention, leaving a potential customer wondering what kind of care or service they’ll get from you.
Sure, you can make the argument that you’re too busy working for your customers to worry about the small details on your own site like that, but then that’s assuming you get the opportunity to make that argument. The potential custom might have already bolted from your site.
Something else you often see on sites is an “page last updated” blurb with the date. Things like these tend only to be important for sites where information frequently gets dated fast. For commercial sites this can be handy if you need to highlight new products added, but generally there are better ways to do that. The best thing to do is to simply make sure that your content remains current and relevant to your products or services.