This module provides functions for expressing durations in rounded or exact
terms.

In the first example in the Synopsis, using duration($interval_seconds):

If the 'time() - $start_time' is 3 seconds, this prints "Runtime: *3
seconds*.". If it's 0 seconds, it's "Runtime: *0 seconds*.". If it's 1
second, it's "Runtime: *1 second*.". If it's 125 seconds, you get "Runtime:
*2 minutes and 5 seconds*.". If it's 3820 seconds (which is exactly 1h, 3m,
40s), you get it rounded to fit within two expressed units: "Runtime: *1
hour and 4 minutes*.". Using duration_exact instead would return "Runtime:
*1 hour, 3 minutes, and 40 seconds*".