diff --git a/DFReader.py b/DFReader.py
index 14821c0da03ed069c628f35e85742b5a2df9c5cc..1e55823304b90f942baccd9ffb1eef9e7ed14868 100644
--- a/DFReader.py
+++ b/DFReader.py
@@ -268,7 +268,7 @@ class DFReaderClock(object):
     def _gpsTimeToTime(self, week, msec):
         '''convert GPS week and TOW to a time in seconds since 1970'''
         epoch = 86400*(10*365 + int((1980-1969)/4) + 1 + 6 - 2)
-        return epoch + 86400*7*week + msec*0.001 - 15
+        return epoch + 86400*7*week + msec*0.001 - 18
 
     def set_timebase(self, base):
         self.timebase = base