Zeit[Startwinkel_, Stablänge_, Schrittlänge_] :=
 Module[{φ_0 = Startwinkel, 
   l = Stablänge, Δt = Schrittlänge},
  ClearAll;
  τ = Sqrt[(2*l)/(3*9.81)];
  φ''[0] = Sin[φ_0]/τ^2;
  t = Round[Δt, Δt];
  φ'[t] = Δt*φ''[0];
  φ[t] = Phi_0 + Δt*φ'[t];
  While[φ[Round[t, Δt]] < Pi/2,
   φ''[Round[t, Δt]] = 
      Sin[φ[Round[t, Δt]]]/τ^2;
   t = Round[t + Δt, Δt];
   φ'[Round[t, Δt]] = 
      φ'[Round[t - Δt, Δt]] + 
      Δt*φ''[Round[t - Δt, Δt]];
   φ[Round[t, Δt]] = 
      φ[Round[t - Δt, Δt]] + 
      Δt*φ'[Round[t, Δt]];
   ];
  t
  ]