Extract constant for "last valid tick in time". We may find we want to use this elsewhere, too.
Extract constant for "last valid tick in time". We may find we want to use this
elsewhere, too.

zaccharles/nodatime,BenJenkinson/nodatime,zaccharles/nodatime,jskeet/nodatime,zaccharles/nodatime,nodatime/nodatime,jskeet/nodatime,zaccharles/nodatime,nodatime/nodatime,malcolmr/nodatime,zaccharles/nodatime,zaccharles/nodatime,malcolmr/nodatime,BenJenkinson/nodatime,malcolmr/nodatime,malcolmr/nodatime