Fix time unit related properties

Fixes a number of bugs and omissions in time unit properties.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5765/