diff --git a/libraries/AP_GPS/AP_GPS_PX4.cpp b/libraries/AP_GPS/AP_GPS_PX4.cpp
index 23ba8aab84..5534067d38 100644
--- a/libraries/AP_GPS/AP_GPS_PX4.cpp
+++ b/libraries/AP_GPS/AP_GPS_PX4.cpp
@@ -39,9 +39,9 @@ AP_GPS_PX4::AP_GPS_PX4(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriv
 }
 
 
-#define MS_PER_WEEK             7*24*3600*1000LL
-#define DELTA_POSIX_GPS_EPOCH   315964800LL*1000LL
-#define LEAP_MS_GPS_2014        16*1000LL
+const uint64_t MS_PER_WEEK            = ((uint64_t)7)*24*3600*1000;
+const uint64_t DELTA_POSIX_GPS_EPOCH  = ((uint64_t)3657)*24*3600*1000;
+const uint64_t LEAP_MS_GPS_2016       = ((uint64_t)17)*1000;
 
 // update internal state if new GPS information is available
 bool
@@ -68,10 +68,10 @@ AP_GPS_PX4::read(void)
 
                 // convert epoch timestamp back to gps epoch - evil hack until we get the genuine
                 // raw week information (or APM switches to Posix epoch ;-) )
-                uint64_t epoch_ms = uint64_t(_gps_pos.time_utc_usec/1000.+.5);
-                uint64_t gps_ms = epoch_ms - DELTA_POSIX_GPS_EPOCH + LEAP_MS_GPS_2014;
-                state.time_week = uint16_t(gps_ms / uint64_t(MS_PER_WEEK));
-                state.time_week_ms = uint32_t(gps_ms - uint64_t(state.time_week)*MS_PER_WEEK);
+                uint64_t epoch_ms = _gps_pos.time_utc_usec/1000;
+                uint64_t gps_ms = epoch_ms - DELTA_POSIX_GPS_EPOCH + LEAP_MS_GPS_2016;
+                state.time_week = (uint16_t)(gps_ms / MS_PER_WEEK);
+                state.time_week_ms = (uint32_t)(gps_ms - (state.time_week)*MS_PER_WEEK);
 
                 if (_gps_pos.time_utc_usec == 0) {
                   // This is a work-around for https://github.com/PX4/Firmware/issues/1474