diff --git a/kcp.go b/kcp.go
index 42c52c16..9d9b7bd4 100644
--- a/kcp.go
+++ b/kcp.go
@@ -485,9 +485,9 @@ func (kcp *KCP) Input(data []byte, regular, ackNoDelay bool) int {
 	}
 
 	var maxack uint32
+	var lastackts uint32
 	var flag int
 	var inSegs uint64
-	current := currentMs()
 
 	for {
 		var ts, sn, length, una, conv uint32
@@ -527,10 +527,6 @@ func (kcp *KCP) Input(data []byte, regular, ackNoDelay bool) int {
 		kcp.shrink_buf()
 
 		if cmd == IKCP_CMD_ACK {
-			if _itimediff(current, ts) >= 0 {
-				kcp.update_ack(_itimediff(current, ts))
-			}
-
 			kcp.parse_ack(sn)
 			kcp.shrink_buf()
 			if flag == 0 {
@@ -539,6 +535,7 @@ func (kcp *KCP) Input(data []byte, regular, ackNoDelay bool) int {
 			} else if _itimediff(sn, maxack) > 0 {
 				maxack = sn
 			}
+			lastackts = ts
 		} else if cmd == IKCP_CMD_PUSH {
 			if _itimediff(sn, kcp.rcv_nxt+kcp.rcv_wnd) < 0 {
 				kcp.ack_push(sn, ts)
@@ -576,6 +573,10 @@ func (kcp *KCP) Input(data []byte, regular, ackNoDelay bool) int {
 
 	if flag != 0 && regular {
 		kcp.parse_fastack(maxack)
+		current := currentMs()
+		if _itimediff(current, lastackts) >= 0 {
+			kcp.update_ack(_itimediff(current, lastackts))
+		}
 	}
 
 	if _itimediff(kcp.snd_una, una) > 0 {