diff --git a/snowflake.go b/snowflake.go
index 7ff668d..cab2d38 100644
--- a/snowflake.go
+++ b/snowflake.go
@@ -61,12 +61,12 @@ func NextID() (uint64, error) {
 		}
 	}
 
-	df := int(elapsedTime(startTime))
+	df := int(elapsedTime(c, startTime))
 	if df < 0 || df > MaxTimestamp {
 		return 0, errors.New("The maximum life cycle of the snowflake algorithm is 2^41-1(millis), please check starttime")
 	}
 
-	id := uint64(df<<timestampMoveLength | machineID<<machineIDMoveLength | int(seq))
+	id := uint64((df << timestampMoveLength) | (machineID << machineIDMoveLength) | int(seq))
 	return id, nil
 }
 
@@ -89,7 +89,7 @@ func SetStartTime(s time.Time) {
 	}
 
 	// Because s must after now, so the `df` not < 0.
-	df := elapsedTime(s)
+	df := elapsedTime(currentMillis(), s)
 	if df > MaxTimestamp {
 		panic("The maximum life cycle of the snowflake algorithm is 69 years")
 	}
@@ -163,8 +163,8 @@ func callSequenceResolver() SequenceResolver {
 	return resolver
 }
 
-func elapsedTime(s time.Time) int64 {
-	return currentMillis() - s.UTC().UnixNano()/1e6
+func elapsedTime(nowms int64, s time.Time) int64 {
+	return nowms - s.UTC().UnixNano()/1e6
 }
 
 // currentMillis get current millisecond.
diff --git a/snowflake_test.go b/snowflake_test.go
index 0870c18..e3693f1 100644
--- a/snowflake_test.go
+++ b/snowflake_test.go
@@ -2,6 +2,7 @@ package snowflake_test
 
 import (
 	"errors"
+	"sync"
 	"testing"
 	"time"
 
@@ -14,6 +15,54 @@ func TestID(t *testing.T) {
 	if id <= 0 {
 		t.Error("The snowflake should't < 0.")
 	}
+
+	id2 := 1644633515267981312
+	df := 392111185853
+	if id2 != ((df << (snowflake.MachineIDLength + snowflake.SequenceLength)) | 0 | 0) {
+		t.Error("Create snowflake should be equal 1644633515267981312")
+	}
+
+	mp := make(map[uint64]bool)
+	for i := 0; i < 100000; i++ {
+		id, e := snowflake.NextID()
+		if e != nil {
+			t.Error(e)
+			continue
+		}
+		if _, ok := mp[id]; ok {
+			t.Error("ID should't repeat", id)
+			break
+		}
+		mp[id] = true
+	}
+}
+
+func TestID_bitch(t *testing.T) {
+	le := 100000
+	ch := make(chan uint64, le)
+	var wg sync.WaitGroup
+	for i := 0; i < le; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			id := snowflake.ID()
+			ch <- id
+		}()
+	}
+	wg.Wait()
+	close(ch)
+
+	mp := make(map[uint64]bool)
+	for id := range ch {
+		if _, ok := mp[id]; ok {
+			t.Error("It should not be repeated")
+			break
+		}
+		mp[id] = true
+	}
+	if len(mp) != le {
+		t.Error("map length should be equal", le)
+	}
 }
 
 func TestSetStartTime(t *testing.T) {