I added your suggestion in [dev.piwik.org]
wow, so great!
It's such a disappointment to not have this basic feature. I just installed Piwik last night and have been pleased with everything I've discovered with it thus far except for this one simple, yet very useful, metric. Please developers: If you can't be bothered to have it show the duration spent on page, AT LEAST have it show a timestamp when the page was visited! C'mon!