A data buffer, such as a data strobe input buffer or a data input buffer, which may operate in multiple modes, such as a single mode (SM) and a dual mode (DM) and where the mode is selected by providing a signal, such as an external signal such as an address signal or an external command signal. A data buffer which can be used for a SM/DM dual-use and can improve a data setup/hold margin. A semiconductor memory device including one or more of the data buffers described above. A method for controlling propagation delay times which can improve a data setup/hold margin in a SM/DM dual-use data buffer.