GrandFather,
I wrote RFC: Seconds2English roughly 1 year after starting to learn perl. While I wrote it as a learning exercise, I actually used it once as a CGI to count down how long before vacation. As JavaFan pointed out, your estimates can be quite a ways off - do you have a real world use for this?
As I hinted in the OP, the initial driver was a very approximate indication of up time for what amounts to a server. The 'tipping' points are intentionally not 'accurate' in any sense but are driven by a feel for a nice transition point from one unit to the next and by intent are not set to change at the 'obvious' points. The second column in the @periods 'table' provides the tipping point data where each number is in the units of the row.