The ED206 and ED406-101 models and the "MASTER" clock for a "MASTER-SLAVE" System default to 12 hour time format.
PLEASE POWER OFF THE UNIT!
A knockout on the back of the enclosure can be removed to access a jumper that is in an 8-pin socket labeled U8 (across pins 7 and 8). Remove the jumper from this position and the display will show 24 hour time. Replace the jumper in U8 (across pins 7 and 8) to display 12 hour format. The knockout to access U8 is on the back of the cabinet.