The milli-seconds since epoch value for an invalid transition is,
of course, invalidMSecs(), not invalidSeconds().
Added a comment while I was at it, explaining why we expect a
transition before the epoch, if such transitions are supported.
Change-Id: I0f376f9d69c0e6e79a309dc011943baa41175135
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>