Bossbar at the top maybe?
This seems like a better idea. An armor stand on which you can click to see the time, or that the time is above the armor stand.Or maybe add an armor stand that tells you when ever you right or left click it
I would rather see a message appear that informs me about this. For example, if you join a lobby, this message appears: ''A new hidden chest will spawn in h/m/s!''