This paper presents a convergence analysis of Crank-Nicolson and Ran-nacher time-marching methods which are often used in finite difference dis-cretisations of the Black-Scholes equations. Particular attention is paid to the important role of Rannacher's startup procedure, in which one or more initial timesteps use Backward Euler timestepping, to achieve second order convergence for approximations of the first and second derivatives. Numer-ical results confirm the sharpness of the error analysis which is based on asymptotic analysis of the behaviour of the Fourier transform. The rele-vance to Black-Scholes applications is discussed in detail, with numerical results supporting recommendations on how to maximise the accuracy for a given computational cost.