When did the 21st century start? 12:00am January 1st 2020 or January 1st 2021? Astronomically speaking, when does tomorrow start? 12:00am GMT (Greenwich Mean Time) or your local time zone. Will changing to Daylight Saving Time March 14 @ 2:00am alter the space-time continuum to a new reality of when “tomorrow” actually begins? Does anyone really care?