diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Changelog for pixela
 
+## 0.3.1.0
+
+- Fix a date format bug
+
 ## 0.3.0.0
 
 ### Breaking Change
diff --git a/pixela.cabal b/pixela.cabal
--- a/pixela.cabal
+++ b/pixela.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 0f11ca651ff1256006127df1c14193e910362ee1ce86a15c5802243a91b335ae
+-- hash: f10be4161b5cf582271bb4505ed58766e07d8e314265f91b057e9b76e6f7573e
 
 name:           pixela
-version:        0.3.0.0
+version:        0.3.1.0
 synopsis:       Pixela client.
 description:    Pixela (<https://pixe.la/>) client library.
 category:       Web
diff --git a/src/Web/Pixela.hs b/src/Web/Pixela.hs
--- a/src/Web/Pixela.hs
+++ b/src/Web/Pixela.hs
@@ -373,7 +373,7 @@
       (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId)
       (Just token')
       ( Just $ Aeson.object
-          [ "date" .= date
+          [ "date" .= formatDate date
           , "quantity" .= quantity
           ]
       )
@@ -391,7 +391,7 @@
 getQuantityBSL graphId date (Client (Config ep _ userName' token') manager) =
   requestBSL
     GET
-    (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId </> URI.encode (formatDay date))
+    (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId </> URI.encode (formatDate date))
     (Just token')
     Nothing
     manager
@@ -403,7 +403,7 @@
   void $
     request
       PUT
-      (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId </> URI.encode (formatDay date))
+      (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId </> URI.encode (formatDate date))
       (Just token')
       (Just $ Aeson.object ["quantity" .= quantity])
       manager
@@ -439,7 +439,7 @@
   void $
     request
       DELETE
-      (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId </> URI.encode (formatDay date))
+      (ep </> "users" </> URI.encode userName' </> "graphs" </> URI.encode graphId </> URI.encode (formatDate date))
       (Just token')
       Nothing
       manager
@@ -544,5 +544,12 @@
 p </> q = p <> "/" <> q
 infixr 6 </>
 
-formatDay :: Day -> String
-formatDay = formatTime defaultTimeLocale "%y%d%m"
+formatDate :: Date -> String
+formatDate day =
+  let
+    (y, m, d) = toGregorian day
+    y' = show y
+    m' = (if m < 10 then "0" else "") <> show m
+    d' = (if d < 10 then "0" else "") <> show d
+  in
+    y' <> m' <> d'