sys: timex cleanup

This PR comprises some improvements for the timex module:
 * better and more documentation
 * some inline functions
 * a more flexible string conversion
Let's see. Oxford (http://www.oed.com/view/Entry/26259?rskey=vPIgt8&result=2&isAdvanced=false#eid) says:
> To estimate or determine by arithmetical or mathematical reckoning; to compute, reckon.

A subtraction is a mathematical reckoning, isn't it?