diff --git a/subsys/net/l2/ethernet/gptp/gptp_mi.c b/subsys/net/l2/ethernet/gptp/gptp_mi.c
index 0859868e8e3..12849add9c9 100644
--- a/subsys/net/l2/ethernet/gptp/gptp_mi.c
+++ b/subsys/net/l2/ethernet/gptp/gptp_mi.c
@@ -778,7 +778,7 @@ static void gptp_update_local_port_clock(void)
 
 	if (second_diff < 0 && nanosecond_diff > 0) {
 		second_diff++;
-		nanosecond_diff = -NSEC_PER_SEC + nanosecond_diff;
+		nanosecond_diff = -(int64_t)NSEC_PER_SEC + nanosecond_diff;
 	}
 
 	ptp_clock_rate_adjust(clk, port_ds->neighbor_rate_ratio);