An aligner with a time switching capability is disclosed. The aligner has a store for storing incoming data. The store is sufficiently large that it can hold more than one frame of information. Data is written into the store under the control of clock signals associated with the incoming data. Data is read from the store under the control of a local clock. Read addresses are generated by a unit which is so arranged that if a clash between the current write address and the generated read address is sensed to be imminent, reading of the corresponding address in the previous or subsequent frame occurs. Thus, the aligner can operate as a time switch and clashes between reading and writing are avoided by allowing slippage of one frame.