Puzzle ZBZL 

Workings

Hint
A fathom is 6 feet.

Answer
Hours in a month.

Reasoning
A fathom is 6 feet, which is 1.8288 metres.

So there are 1000 ÷ 1.8288 ≈ 546.8 fathoms in a kilometre.

The shortest month has 28 x 24 hours = 672.

So even the shortest month has more hours than there are fathoms in a kilometre.