If you have a look here you'll find some functions for time formatting ZOs uses ingame:
https://github.com/esoui/esoui/blob/...obals/time.lua
I think the function here fits your needs:
function ZO_FormatCountdownTimer(seconds)
Maybe you can find in the addon the code where the time left is calculated and you can change it to use this function. The functions uses as parameter the seconds and you need to use it like this:
Lua Code:
--Assuming the seconds left in the addon is stored in variable "secondsLeft"
local timeLeft = ZO_FormatCountdownTimer(secondsLeft)
--Then use your variable "timeLeft" for the output.