Proposal for an elementary "time axis" package.<br/>Proposal includes:<br/><ul>
<li>instants: no classes, but a DateTime type (instants are values of type DateTime);</li><li>continuous time intervals: class SimpleTimeInterval, including or not each extremity;</li><li>composite time intervals: a simple mechanism to add or subtract simple (or other composite) intervals.</li></ul>
<br/>The abstract class TimeInterval is not only a element of the "composite" pattern; it shall also be referenced systematically by other classes, whenever the "simple" or "composite" nature of the interval cannot be decided.<br/>Operations on time intervals are left to implementations (PSM).<br/>