Why do external Java libraries paths have to be refreshed in eclipse between starts?

For my school projects, I use the joda-time API for generating timestamps that are used for file names and logging.

Getting eclipse to recognize the library is no problem. However, when I start eclipse between reboots, I get the red X in the project tree because the external API cannot be found, even though the file path has not changed. So each time, I have to go to the Libraries tab in Build Path to re-target the API. Frankly, this is getting to be a PI开发者_JS百科MA.

So is there any way to have the path be permanent, so I don't have to do the Build Path rigmarole all the time?