This has been a long-standing issue in Moodle. I have learned from Martin that originally course home pages were always in weeks format and all dates were relative to the week in which the activity was placed. However this was dropped when the topics format was introduced. This feature of relative dates is now still waiting to be re-introduced.
The other feature that you request, namely the option to make dates optional is also still waiting to be implemented for all activity modules with dates. Currently your only option is to put the start dates in the far past and the closing dates in the far future. This is not really satisfactory.
As an interim workaround you may be interested in the date management screen, see http://moodle.org/mod/forum/discuss.php?d=9697.