Use CST when referring to time in the central time zone when and where Daylight Saving Time is not in effect. Use CDT when referring to time in the central time zone when and where Daylight Saving Time is in effect. Use CT to refer to Connecticut, or any number of other things abbreviated CT, but NEVER to identify a time zone, unless your goal is to confuse people. CST is Central Standard Time. CDT is Central Daylight Time. CT is Connecticut, but some people mistakenly use it as an abbreviation for Central Time. Daylight Saving Time begins and ends on dates set by law.