M The DST column shows the months in which Daylight Saving Time, a.k.a. Summer Time, begins and ends. A blank DST box usually indicates that the location stays on Standard Time all year, although in some cases the location stays on Summer Time all year. If a location is currently on DST, add one hour to the time in the Time column. There are 645 IATA airport codes beginning with M, to be compared with the 676 (26*26) theoretical combinations of the two other letters.