Responses
After the first time Lee Hyunmook mispoke about time, i notice it too. But he decided to hide it from Yohan
In the novel its talks about more.
He hides it because Yohan doesn’t know of the time difference, to him 1 day in the abyss is 1 day in the real world.
Remember that 1 year in the abyss is 15 days in real life. Lee Hyunmook and his team was there for 3+ years irl. So, they've been stuck there for 72 - 90 years.