Rather than just a clock ticking on the wall, Time in "Fern Hill" is almost like a character. Thomas personifies time throughout the poem, as something with immense power. At first, he's the guy who lets the speaker frolic, all happy-like among the meadows. But then time becomes the one who yanks our young and carefree speaker out of his graceful youth and into ugly adulthood. In other words, despite his strong start, time winds up being a real jerk.
In "Fern Hill," the speaker's relationship with time pulls the old switcheroo and shifts from a feeling of being set free to a feeling of being chained by time's inevitable passing. Sneaky little bugger, ain't he?
Time flies when you're having a good time, but luckily for us, the speaker avoids using cliché phrases like that. Instead, he uses natural images, such as rivers, streams and the sea to embody his shifting understanding of time as something beautiful but also fleeting.