Google Analytics uses JavaScript and cookies to record all of the stats on your website. JavaScript by nature is static, as in once its been run it stops and waits for more input(mouse click, item change, etc...). With this in mind when someone comes to your page with Google Analytics on it their browser loads the Google Analytics code which is turns does its cookie/JavaScript magic and records all the statistical information about the visit. Once the stat has been recorded the script stops and waits for more input, in this case another page load(or link click). The reason you see 00:00 as the time spent on the page is because Google Analytics never got that second page load it was looking for, instead the user left your site and Google Analytics was unable to calculate their time on your site. To understand this better I will provide you with a little background on how the Google Analytics code works.
When someone hits your page part of the JavaScript code records the time. When your guest clicks on an internal link that contains Google Analytics code the time is recorded again. Google Analytics then realizes it's the same user(session) and they take the second time, subtract the first time and they now know how long the user was on the previous page. This time is recorded and viewable in the Google Analytics administration section. The problem comes with a user doesn't click another internal link or a tracked external link on your site. This happens usually because the landing page was not interesting enough or did not have relevant information and they decided to leave your site(Not good).
There are two common ways for this to happen. Probably the more common way is the user decides this page does not have the content they were searching for so they hit the back button. When the guest first came to your site Google Analytics recorded the hit along with the date and time. Now, when the user leaves your site using the back button there is no way for Google Analytics to record the hit, thus they have no idea how long the guest was on your site and therfor you get the basic 00:00. There is no way that I know of to correct this issue besides making your guests want to continue viewing you site. The second common way that the time shows up as zero's is when users clicks on an external link(a link pointing to another site). If your page contains external links that are not tagged with Google Analytics code there is no way for Google Analytics to record when that user leaves your site and if they don't know when they left they don't know how long they stayed. Luckily you can fix this easily enough by adding tracking code to your outbound links, see How do I track clicks on outbound links? for more information.
So, in short the reason you see 00:00 for the time spent on your site is probably because people are leaving. They are leaving either through untagged outbound links or by hitting the back button, either way it's a bad sign. So, how do you fix this? First, make sure all of your outbound links are tagged correctly using Google Analytics code. And second, write more relevant content so the people don't want to leave your site. Good luck and happy tracking.