In this letter, we consider the asynchronous detection of diffusion-based molecular communications in timing modulation channel. We propose a symbol detection algorithm that can be used in asynchronous systems. Using the variance of the arrival time of a large number of information molecules arriving at a receiver, it is possible to estimate an average propagation time and the emitting time of each bit from a transmitter, and a transmitted bit sequence can also be decoded. The error probability of the proposed algorithm is analyzed, and it is shown that there is good agreement between analysis and simulation results.