It's mentioned on several occasions earlier in the book that Wen froze the valley's garden into what he considers to be a single perfect moment:
A few cherry blossom petals drifted down onto Wen's head from one of the trees that grew wild along the streamlets.
“And this perfect day will last for ever,” he said. “The air is crisp, the sun is bright, there is ice in the streams. Every day in this valley will be this perfect day.”
“Could get a bit repetitive, master,” said Clodpool.
Lobsang's gift to his master (someone who's spent more than 800 years of his unnaturally long life sweeping up the blossoms) was to finally allow the trees to ripen, giving him the greatest gift of all; a genuine surprise.
A breeze blew the scent of cherry blossom. Just once, thought Lu-Tze,
it would be nice to pick cherries.
Lu-Tze had been right, Lobsang knew. Time was a resource. You could
learn to let it move fast or slow, so that a monk could walk easily
through a crowd and yet be moving so fast that no one could see him.
Or he could stand still for a few seconds, and watch the sun and moon
chase one another across a flickering sky. He could meditate for a day
in a minute. Here, in the valley, a day lasted for ever. Blossom never
Hat tip to @Murphy for finding a better quote