This is going to be a tutorial that explains: How to set up a Countdown timer with Command Blocks and a Hopper Clock setup.

I'll be providing the basic setup to get it started and how I use the setup on my own realm.

First things first you need to get a command block.

/give @p minecraft:command_block 1



Next we're going to use the Scoreboard command to create the variable Timer. This will hold the amount of time you want in seconds



/scoreboard objectives add Timer dummy "Timer"

When using the Scoreboard command Timer is the variable you will use when you want to add or subtract time from you display. dummy makes it so the only to add or subtract from Timer is by using commands. "Timer" is what the display name will show up as. You can change the display name to what you want you just have to make sure you use quotation marks.

Next we're still going to be using Scoreboard but now we're going to make it display Timer by using setdisplay sidebar Timer



/scoreboard objectives setdisplay sidebar Timer

Timer and Timer are the exact same thing don't let the color change confuse you. Once you use this command you will need to do the next command to see a box on the side of your screen labelled Timer, your name and and a numerical value that is stored in Timer.

This is what it will look like once we set Timer with a value



Now we need to set Timer with a numerical value we'll do 60 for 60 seconds. This is the Scoreboard command you're going to use.



/scoreboard players set @p Timer 60

Just a few more steps and you will have a functioning countdown timer. Next we're going to setup a Hopper Clock that will remove 1 from Timer every second.

You will need:
2 Hoppers
2 Comparators
2 Repeaters
8 Redstone Dust
2 Sticky Pistons (MUST BE STICKY)
1 Redstone Block
1 Command Block
3 Random items (Make sure its all the same i.e. 3 Oak Logs or 3 Raw Cod or 3 Dirt. Whatever you want it to be.)



To see how to setup the Hopper Clock here is a video. Credit to Wattles

Once you have it setup make sure to place a Redstone Dust on top of your command block by holding the Shift Key and Right Clicking the top of the Command block.

Next Place your 3 random items into the hopper and your clock. 3 Items will activated your Command Block every second.

Lastly the Command to remove 1 second every time your Command Block is activated.



/scoreboard player remove @p Timer 1

Once you have everything setup and running this is what it should look like. (Sorry for the potato quality gif)



The only command block you need for this setup is the one shown removing time in the Hopper Clock Setup. But you can put all of the commands in command blocks if you like having clear instructions on what each block does.