Put these in chat:
/scoreboard objectives add Timer dummy
/scoreboard objectives setdisplay sidebar Timer
/scoreboard objectives add time2 dummy

Put these in the line of command blocks:
/scoreboard players add @a time2 1
/scoreboard players remove @a[score_time2_min=20] Timer 1
/scoreboard players reset @a[score_time2_min=20] time2
/execute @a[score_Timer=0,score_Timer_min=0] ~ ~ ~ /say @a Timer Ended
/scoreboard players reset @a[score_Timer=0] Timer

This command will start the timer:
/scoreboard players set @a Timer 30