The physics
A coil of N turns spins at ω between two magnet poles. Start the clock with its plane across B. The flux through it is then N B A cos ωt.
Φ = N B A cos ωt ε = N B A ω sin ωt
The emf peaks at ε₀ = N B A ω. It is largest when the coil's plane lies along B. There the flux is zero, but it changes fastest.
The flux and the emf peak a quarter turn apart. With the plane at α to B, the emf is ε₀ cos α.
ε = ε₀ cos α ω = 2πf = 2π × rpm / 60
One full turn makes one cycle of AC. So the frequency in hertz is the number of turns each second.