The present invention relates to a device and method for measuring the differential delay in a computer system having a disaster recovery secondary site. The device includes a transmitter for use at a primary site, the transmitter having a first laser and a second laser. The first laser is optically connected to an end of the transmission path and the second laser is optically connected to an end of the receive path. A receiver is located at the secondary site. The receiver has a first optical receiver optically connected to an end of the transmission path and a second optical receiver optically connected to an end of the receive path. The receiver includes a microprocessor to count the number of cycles between the receipt of light pulses simultaneously emitted from the first and second lasers. From this cycle count, the differential delay between the transmission and receive path is calculated.