system time is not updated at boot up from GPS

[abaz]

Hello,
I am checking your application for timestamping of events from adafruit GPS with PPS outout, I have managed to run your program successfully but I noticed that when I turn off the Rasberry Pi and turn it on again say 4 hours later, the date time restarts from the last datetime it was on (in this case 4 hours off). I have setup the system so uart is enabled in pps-client to get date time from GPS instead of network.
when I do cat /dev/ttyS0 I see the correct time in NMEA lines, but for some reason the pps-client does the 1 second counting from the last date time. Once rasberry pi is on the counting of datetime is as usual, expect it's offset due to time it was off. Any thoughts of what could be happening is greatly appreciated! Thanks!