diff --git a/internal/server/auth.go b/internal/server/auth.go
index 40eae1b..9cf0e70 100644
--- a/internal/server/auth.go
+++ b/internal/server/auth.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"github.com/cbeuw/Cloak/internal/ecdh"
 	"github.com/cbeuw/Cloak/internal/util"
+	"time"
 )
 
 var ErrReplay = errors.New("duplicate random")
@@ -50,7 +51,9 @@ func TouchStone(ch *ClientHello, sta *State) (UID []byte, sessionID uint32, prox
 	proxyMethod = string(bytes.Trim(plaintext[16:28], "\x00"))
 	encryptionMethod = plaintext[28]
 	timestamp := int64(binary.BigEndian.Uint64(plaintext[29:37]))
-	if timestamp/int64(TIMESTAMP_WINDOW.Seconds()) != sta.Now().Unix()/int64(TIMESTAMP_WINDOW.Seconds()) {
+	clientTime := time.Unix(timestamp, 0)
+	serverTime := sta.Now()
+	if !(clientTime.After(serverTime.Truncate(TIMESTAMP_TOLERANCE)) && clientTime.Before(serverTime.Add(TIMESTAMP_TOLERANCE))) {
 		err = fmt.Errorf("%v: received timestamp %v", ErrTimestampOutOfWindow, timestamp)
 		return
 	}
diff --git a/internal/server/state.go b/internal/server/state.go
index 6f80a44..0af7d4f 100644
--- a/internal/server/state.go
+++ b/internal/server/state.go
@@ -121,20 +121,18 @@ func (sta *State) IsBypass(UID []byte) bool {
 	return exist
 }
 
-// This is the accepting window of the encrypted timestamp from client
-// we reject the client if the timestamp is outside of this window.
-// This is for replay prevention so that we don't have to save unlimited amount of
-// random
-const TIMESTAMP_WINDOW = 12 * time.Hour
+const TIMESTAMP_TOLERANCE = 180 * time.Second
+
+const CACHE_CLEAN_INTERVAL = 12 * time.Hour
 
 // UsedRandomCleaner clears the cache of used random fields every 12 hours
 func (sta *State) UsedRandomCleaner() {
 	for {
-		time.Sleep(TIMESTAMP_WINDOW)
-		now := sta.Now().Unix()
+		time.Sleep(CACHE_CLEAN_INTERVAL)
+		now := sta.Now()
 		sta.usedRandomM.Lock()
 		for key, t := range sta.usedRandom {
-			if now-t > int64(TIMESTAMP_WINDOW.Seconds()) {
+			if time.Unix(t, 0).Before(now.Add(TIMESTAMP_TOLERANCE)) {
 				delete(sta.usedRandom, key)
 			}
 		}