As electrical power is applied, the sensor starts at the top of the hour and reports the elapsed time in microseconds past the top of the hour.
When it gets a NMEA message and PPS signal, it synchronizes the top of the hour counter to the time in the NMEA record.
FROM Garmin GPS18LV Manual
The rising edge of the Garmin's PPS signal is aligned to the start of each GPS second within 1 μs for all conditions in which the receiver has reported a valid and accurate position for at least the previous 4 seconds.
The NMEA 0183 sentences that follow each rising edge of the PPS signal tell when you were and where you were at that previous rising edge of the PPS signal, beginning with the GPRMC sentence as the lead sentence in any particular NMEA 0183 record.
Regardless of the selected baud rate, the information transmitted by the GPS 18x series products is referenced to the pulse immediately preceding the NMEA 0183 RMC sentence.
The accuracy of the one-pulse-per-second output is maintained only while the GPS receiver is computing a valid position fix. To obtain the most accurate results, the one-pulse-per-second output should be calibrated against a local time reference to compensate for cable and internal receiver delays and the local time bias.
The default pulse width is 100 ms, however; it may be programmed in 20 ms increments between 20 ms and 980 ms as described in $PGRMC section 4.1.2 Sensor Initialization Information (PGRMI), field <13>