phase φ : state of the motion after last passing through the central position (in positive direction)
φ = t/T
This formula as at t = 0 s the phase= 0 is. This means at t = 0 s the body is moving in a positive direction through the central position.
A body is in simple harmonic motion. f = 5.0 Hz .
At t=0 s the body moves trough the central position in positive direction ( φ (0) = 0 )
At t = 0.34 s : φ = 0.34/0.20 = 1.7 = 0.7