diff --git a/session/common.go b/session/common.go
index 901a68d..745d979 100644
--- a/session/common.go
+++ b/session/common.go
@@ -6,7 +6,6 @@ import (
 	"encoding/hex"
 	"errors"
 	"math/rand"
-	"strconv"
 	"strings"
 	"time"
 
@@ -18,20 +17,21 @@ type Authorization struct {
 	invalidated string
 
 	// by authorization provider
-	Platform    string `bson:"p" json:"p"`
-	Uid         string `bson:"u" json:"u"`
-	Alias       string `bson:"al" json:"al"`
-	CreatedTime int64  `bson:"ct" json:"ct"`
+	Platform    string             `bson:"p" json:"p"`
+	Uid         string             `bson:"u" json:"u"`
+	Alias       string             `bson:"al" json:"al"`
+	CreatedTime primitive.DateTime `bson:"ct" json:"ct"`
 }
 
 func (auth *Authorization) ToStrings() []string {
+	ct, _ := auth.CreatedTime.MarshalJSON()
 	return []string{
 		"a", auth.Account.Hex(),
 		"p", auth.Platform,
 		"u", auth.Uid,
 		"al", auth.Alias,
 		"inv", auth.invalidated,
-		"ct", strconv.FormatInt(auth.CreatedTime, 10),
+		"ct", string(ct),
 	}
 }
 
@@ -41,14 +41,15 @@ func (auth *Authorization) Valid() bool {
 
 func MakeAuthrizationFromStringMap(src map[string]string) Authorization {
 	accid, _ := primitive.ObjectIDFromHex(src["a"])
-	ct, _ := strconv.ParseInt(src["ct"], 10, 0)
+	var datetime primitive.DateTime
+	datetime.UnmarshalJSON([]byte(src["ct"]))
 	return Authorization{
 		Account:     accid,
 		Platform:    src["p"],
 		Uid:         src["u"],
 		Alias:       src["al"],
 		invalidated: src["inv"],
-		CreatedTime: ct,
+		CreatedTime: datetime,
 	}
 }