diff --git a/.gitignore b/.gitignore
index b6e4761..6fe11b1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+/learn
+
 # Byte-compiled / optimized / DLL files
 __pycache__/
 *.py[cod]
diff --git a/downloader.py b/downloader.py
index 2bc68f7..6d201d9 100644
--- a/downloader.py
+++ b/downloader.py
@@ -9,7 +9,7 @@
 
 class Downloader(LearnHelper):
     prefix: str = ""
-    file_size_limit: int = None
+    file_size_limit: int = None # MB
     sync_docs: bool = True
     sync_work: bool = True
     sync_submit: bool = True
@@ -34,12 +34,13 @@
 
     def Download(self, url: str, prefix: str, filename: str) -> bool:
         os.makedirs(prefix, exist_ok=True)
-        filename = slugify(filename)
         response = self.get(url=url, stream=True)
         file_size = int(response.headers.get("content-length", 0))
         if self.file_size_limit:
-            if file_size > self.file_size_limit * 1024:
+            if file_size > self.file_size_limit * 1024 * 1024:
+                print(f"Skipping file {filename}")
                 return False
+        filename = slugify(filename)
         chunk_size = 8192  # 8KB
         with tqdm(
             desc=filename,
@@ -122,7 +123,7 @@
         lines.append(f"")
         if homework.attachment:
             filename = slugify(
-                f"{homework.title}{os.path.splitext(homework.attachment.name)[-1]}"
+                f"attach-{homework.title}{os.path.splitext(homework.attachment.name)[-1]}"
             )
             self.Download(
                 url=homework.attachment.download_url, prefix=prefix, filename=filename,
@@ -150,7 +151,7 @@
             lines.append(f"")
         lines.append(f"### Deadline (GMT+8)")
         lines.append(f"")
-        lines.append(f"{homework.deadline}")
+        lines.append(f"{homework.deadline.isoformat() if homework.deadline else None}")
         lines.append(f"")
         if self.sync_submit:
             lines.append(f"## My coursework submitted")
@@ -180,7 +181,9 @@
             lines.append(f"")
             lines.append(f"### Date")
             lines.append(f"")
-            lines.append(f"{homework.grade_time}")
+            lines.append(
+                f"{homework.grade_time.isoformat() if homework.grade_time else None}"
+            )
             lines.append(f"")
             lines.append(f"### Grade")
             lines.append(f"")
diff --git a/main.py b/main.py
index 60cca64..f4e7ea2 100644
--- a/main.py
+++ b/main.py
@@ -17,14 +17,17 @@
     help="semesters to be synced. If you want to sync all semesters, do not pass this argument",
 )
 parser.add_argument(
-    "--prefix", default=None, required=False, help="location to save downloaded files",
+    "--prefix",
+    default=None,
+    required=False,
+    help='location to save downloaded files, default to "learn/"',
 )
 parser.add_argument(
     "--file_size_limit",
     default=None,
     type=float,
     required=False,
-    help="files exceed limit will not be downloaded",
+    help="(unit: MB) files exceed limit will not be downloaded",
 )
 parser.add_argument(
     "--no_sync_docs",
diff --git a/requirements.txt b/requirements.txt
index 38b4e19..1090c17 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,4 @@
 beautifulsoup4~=4.11.1
-requests~=2.27.1
+requests~=2.28.1
 tqdm~=4.64.0
 python-slugify~=6.1.2
diff --git a/thu_learn_lib/helper.py b/thu_learn_lib/helper.py
index e5a80b0..b78d92e 100644
--- a/thu_learn_lib/helper.py
+++ b/thu_learn_lib/helper.py
@@ -218,15 +218,21 @@
                         student_homework_id=work["xszyid"],  # 学生作业 ID
                     )
                 ),
-                deadline=work["jzsj"],  # 截止时间
+                deadline=datetime.datetime.fromtimestamp(work["jzsj"] / 1000.0)
+                if work["jzsj"]
+                else None,  # 截止时间
                 submit_url=urls.learn_homework_submit(
                     work["wlkcid"], work["xszyid"]  # 课程 ID, 学生作业 ID
                 ),
-                submit_time=work["scsj"],  # 上传时间
+                submit_time=datetime.datetime.fromtimestamp(work["scsj"] / 1000.0)
+                if work["scsj"]
+                else None,  # 上传时间
                 grade=work["cj"],  # 成绩
                 grader_name=work["jsm"],  # 教师名
                 grade_content=work["pynr"],  # 批阅内容
-                grade_time=work["pysj"],  # 批阅时间
+                grade_time=datetime.datetime.fromtimestamp(work["pysj"] / 1000.0)
+                if work["pysj"]
+                else None,  # 批阅时间
                 submitted=status.submitted,
                 graded=status.graded,
                 description=detail.description,