I did this by setting up a timer in HA (Countdown kitchen timer - #15 by Flipso) and then displaying it. I press a tile in Tileboard to activate the HA automation. Once it is active another tile becomes visible in Tileboard and shows the timer. When tmer is finished lights flash and the tile is hidden again. My child uses this to time how long until their cup of tea is ready.
As an aside before Google broke the integration with HA I was using a similar approach to display the current status of my Google timers in TielBoard - which was really useful.)
{
position: [0, 3], // [x, y]
width: 1,
type: TYPES.SENSOR,
title: 'Tea Timer running',
id: 'counter.timer_counter',
customStyles: {'color': '#d469f4','font-weight':'900'},
state: false,
hidden:function(item, entity){
if (entity.state==240) {return true}
else {return false}
},
filter: function (value) {
var minutes = Math.floor(value / 60);
var seconds = value - minutes * 60;
var final;
if(seconds<=9){ final = minutes +':0'+seconds;}
else{ final = minutes +':'+seconds;}
return final;
}