Fantasy Problems: How to Measure Time Underground

This week I ran into an interesting snag as I wrote The Moonstone: How do you keep track of time while you are underground?
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Here's the background: Katryl and her companions accidentally wind up in Undercountry, a vast kingdom under the earth whose inhabitants have not seen the sun for a thousand years. There are no days and no nights, no natural wake and sleep cycles, no seasons, no celestial bodies to mark the passage of time--nothing by which we would normally keep time.

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Since there are no traditional time-keeping methods, I can not indicate how long my Overdwellers have traveled in Undercountry. "We encountered the water-snake three days ago" just does not compute when there are no days.

So how does one keep time while stuck underground?

This is the idea that I eventually came up with, illustrated by a conversation between Edric, an Overdweller and one of Katryl's companions, and Rory, a Dwarf who has never visited Overcountry:

"That is something I would like to know," Edric said. "How do you measure time here in Undercountry? In Overcountry, we measure time by the dimming and brightening of the light and by the changes of the moon."

"I will not even ask what a moon might be. The first thing that comes to mind is a creature whose coat changes with the seasons... Ah, but I can see by your expression that it is not so. Shut up."

"I did not say anything."

"You were thinking it, which is just as bad."

"The Undercountry seasons, Rory."

"They are determined by the growth of the hoochard, which grows all throughout Undercountry, even in the Troll areas. You have probably seen it before. It is a low-growing plant, pale in color..."

"The glowing moss?"

"Great hammers, man! That is pzakar. If you ever try to eat it, your eyes will bleed and your insides will disintegrate."

"Truly?"

"No," Rory grinned. "But I liked the flare of alarm that rose in your eyes just now."

"You are vastly irritating."

Rory laughed outright and clapped Edric in the middle of his back. Edric choked.

"My apologies," Rory said. "But your shoulder was too high."

Edric's hand struck Rory's head without warning and the Dwarf staggered before righting himself.

"My apologies," Edric said stiffly. "But your shoulder was too low."

Rory observed his companion's expression, then burst into a raucous peal of laughter. "Ha! Edric of nowhere, you are the most insulting son of a senile dragon that I ever met!"

"And you are the most vexing, topic-bouncing son of a half-witted pirate that I ever met. The seasons of hoochard, if you please."

"Ah! The hoochard. It is a pale, low-growing plant distinguished by its succulent, fleshy oval leaves and pale pink vines. It covers great quantities of the ground in all parts of Undercountry, and can keep a man alive for many days. It goes through four seasons, however, turning first blue, then green, then yellow, then pink. Each of the seasons influences its taste and texture slightly. We count time by those seasons. If something was done 'during the yellow season,' it was done during the time when the hoochard grew yellow."

"I see," Edric nodded. "And are there ways to measure time within a season? There must be a more precise system by which you carry out campaigns and coordinate meetings."

"We carry time-pieces," Rory replied and, reaching into the pocket of his roughly-woven trousers, drew out a small oval device on a chain. "The cogs and wheels are hidden behind the face of it, but you can see the three moving needles. Each awaking is marked by a black diamond in the face. The large needle makes one revolution from awaking to awaking, the mid-sized needle makes ten revolutions from awaking to awaking, and the small needle makes a hundred revolutions from awaking to awaking. We call such revolutions longs, mids, and shorts."

Edric stopped at once and bent over the device, turning it over and over in his hands. Rory obliged by removing the face of the time-piece through the means of a small, spring-operated button, and Edric poured breathlessly over the tiny moving cogs and wheels. 
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"How do you create workmanship so fine?" Edric asked at last.
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"Dwarves are excellent craftsmen," Rory replied. "Though timepieces are a specialty. When we arrive at the city, I will show you the time-piece crafters. They would be most flattered to entertain a guest as ignorant as you."

What do you think of this idea? Is it believable? Is it clear or do you need more description?

​If you have an idea that you think is just killer, let me know!