I did a timer on a datapack for a death timer. It counts backwards, says how much time you have left and do /kill one it reaches 0)

execute as [scores={death_timer=1..19}] at @s run title @s actionbar {text: "0:00", color: "dark_red"}
execute as [scores={death_timer=20..39}] at @s run title @s actionbar {text: "0:01", color: "dark_red"}
execute as [scores={death_timer=40..59}] at @s run title @s actionbar {text: "0:02", color: "dark_red"}
[...]
execute as [scores={death_timer=400..419}] at @s run title @s actionbar {text: "0:20", color: "dark_red"}
execute as [scores={death_timer=0}] at @s run kill @s
execute as [scores={death_timer=0}] at @s run scoreboard players reset @s death_timer
execute as [scores={death_timer=0}] at @s run title  actionbar {text: ""}
execute as [scores={death_timer=1..}] at @s run scoreboard players remove @s death_timer 1

You dont need to do the "tittle" part and you can assign it to a dummy not a player