Why Time Doesn't Convert in Clean Powers of Ten
Unlike metric length or mass units, time durations don't follow a consistent base-10 (or any single consistent base) scaling — seconds to minutes is a factor of 60, minutes to hours is another factor of 60, hours to days is a factor of 24, and days to years introduces yet another irregular factor (365.25, accounting for leap years) — a genuinely inconsistent mix of bases inherited from historical astronomical and Babylonian sexagesimal (base-60) conventions, not a designed system.
This irregularity is exactly why converting a duration across several of these units by mental math accumulates error easily — going from seconds to years involves multiplying through several different, non-round factors in sequence, and a small arithmetic slip at any step compounds through the rest.
The 365.25-day year (accounting for the roughly quarter-day Earth's orbit doesn't neatly fit into 365 whole days) is itself an approximation smoothed over by leap years — which is why "exactly how many seconds are in a year" doesn't have one single universally exact answer without first specifying whether leap years are being averaged in or a specific calendar year is meant.