The most accurate option is using the hardware PPS (pulse per second) signal with the PPS message (http://docs.carnegierobotics.com/libmultisense/classcrl_1_1multisense_1_1pps_1_1_header.html). The PPS output line is part of the M12 power connector. Once every second, the camera pulses the PPS line and simultaneously sends a message over Ethernet containing the current MultiSense time. The computer must have a hardware IO interface that can run an interrupt program when it receives the PPS signal. After receiving the PPS signal, the computer must measure the elapsed time until receiving the PPS timestamp message over Ethernet. You can use this latency measurement to either synchronize your system time with the MultiSense clock, or convert MultiSense time stamps into your system times.
We have a slightly less accurate option, which is enabled by default in LibMultiSense. It is called network time sync (http://docs.carnegierobotics.com/libmultisense/classcrl_1_1multisense_1_1_channel.html#aa4b20ea71b6f040278cb877ee0dc5ad8). It has the advantage of not requiring the computer to have hard IO. When enabled, sensor timestamps, such as the image header timestamp, will be reported in the local system clock frame (gettimeofday() function).
The third option is to use triggers to take images. When configured to use triggers, the cameras will only capture images when they see a signal on the trigger input of the M12 power connector. This can be used to fire multiple cameras simultaneously, and may eliminate the need to know the MultiSense system time because the computer determined when the images should be captured.
The MultiSense S21B M12 pin assignments can be found on this document: https://s3-us-west-2.amazonaws.com/files.carnegierobotics.com/products/MultiSense_S21B/MULTISENSE-S21B.PDF
A bit more detail about the IO interface with the camera can be found here: https://s3-us-west-2.amazonaws.com/files.carnegierobotics.com/products/MultiSense_S21B/MULTISENSE-S21B.PDF (the drawing references our old-style 9-19 MIL Connector, but the electrical specifications are the same when using the M12 connector)