Are you sure DotA does not use a timer dialog? As for the multiboard countdown that is done by refreshing the leaderboard items every second with a new time.
For a count down loop you will need to make your own "for loop". This is done using JASS.
An example using loop index A...
set bj_forLoopAIndex = 20
loop
// your code here
exithwhen bj_forLoopAIndex <= 1
set bj_forLoopAIndex = bj_forLoopAIndex - 1
endloop
For a count down loop you will need to make your own "for loop". This is done using JASS.
An example using loop index A...
set bj_forLoopAIndex = 20
loop
// your code here
exithwhen bj_forLoopAIndex <= 1
set bj_forLoopAIndex = bj_forLoopAIndex - 1
endloop