diff --git a/aiogram/api/session/base.py b/aiogram/api/session/base.py
index f0e6eefe..113af58e 100644
--- a/aiogram/api/session/base.py
+++ b/aiogram/api/session/base.py
@@ -2,7 +2,7 @@ import abc
 import asyncio
 import datetime
 import json
-from typing import Any, Callable, Dict, List, Optional, TypeVar, Union
+from typing import Any, Callable, Optional, TypeVar, Union
 
 from pydantic.dataclasses import dataclass
 
@@ -76,13 +76,13 @@ class BaseSession(abc.ABC):
             return self.json_dumps(self.clean_json(value))
         if isinstance(value, datetime.timedelta):
             now = datetime.datetime.now()
-            return int((now + value).timestamp())
+            return str(round((now + value).timestamp()))
         if isinstance(value, datetime.datetime):
-            return round(value.timestamp())
+            return str(round(value.timestamp()))
         else:
             return str(value)
 
-    def clean_json(self, value: Union[List, Dict]):
+    def clean_json(self, value: Any):
         if isinstance(value, list):
             return [self.clean_json(v) for v in value if v is not None]
         elif isinstance(value, dict):
diff --git a/poetry.lock b/poetry.lock
index 07661b9c..a04eb8c9 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -49,6 +49,14 @@ optional = false
 python-versions = ">=3.5.3"
 version = "3.0.1"
 
+[[package]]
+category = "dev"
+description = "Enhance the standard unittest package with features for testing asyncio libraries"
+name = "asynctest"
+optional = false
+python-versions = ">=3.5"
+version = "0.13.0"
+
 [[package]]
 category = "main"
 description = "Atomic file writes."
@@ -109,7 +117,7 @@ version = "7.0"
 [[package]]
 category = "main"
 description = "Cross-platform colored terminal text."
-marker = "sys_platform == \"win32\""
+marker = "python_version >= \"3.5\" and sys_platform == \"win32\" or sys_platform == \"win32\""
 name = "colorama"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
@@ -164,7 +172,7 @@ version = "2.8"
 [[package]]
 category = "main"
 description = "Read metadata from Python packages"
-marker = "python_version < \"3.8\""
+marker = "python_version >= \"3.5\" and python_version < \"3.8\" or python_version < \"3.8\""
 name = "importlib-metadata"
 optional = false
 python-versions = ">=2.7,!=3.0,!=3.1,!=3.2,!=3.3"
@@ -457,6 +465,38 @@ version = "2.8.1"
 coverage = ">=4.4"
 pytest = ">=3.6"
 
+[[package]]
+category = "main"
+description = "Thin-wrapper around the mock package for easier use with py.test"
+name = "pytest-mock"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+version = "1.11.2"
+
+[package.dependencies]
+pytest = ">=2.7"
+
+[[package]]
+category = "dev"
+description = "Mypy static type checker plugin for Pytest"
+name = "pytest-mypy"
+optional = false
+python-versions = "~=3.4"
+version = "0.4.2"
+
+[package.dependencies]
+[[package.dependencies.mypy]]
+python = ">=3.5,<3.8"
+version = ">=0.500"
+
+[[package.dependencies.mypy]]
+python = ">=3.8"
+version = ">=0.700"
+
+[package.dependencies.pytest]
+python = ">=3.5"
+version = ">=2.8"
+
 [[package]]
 category = "main"
 description = "World timezone definitions, modern and historical"
@@ -544,7 +584,7 @@ multidict = ">=4.0"
 [[package]]
 category = "main"
 description = "Backport of pathlib-compatible object wrapper for zip files"
-marker = "python_version < \"3.8\""
+marker = "python_version >= \"3.5\" and python_version < \"3.8\" or python_version < \"3.8\""
 name = "zipp"
 optional = false
 python-versions = ">=2.7"
@@ -554,7 +594,7 @@ version = "0.6.0"
 more-itertools = "*"
 
 [metadata]
-content-hash = "5252c850bdd8bc1c88cfb7aedc2ce27896fc06310a0ecf6674548702051cadcc"
+content-hash = "36e7679dedd3bcc50c24575b35da90a64bd33ef54d7862db94159dfc7ecaf28a"
 python-versions = "^3.7"
 
 [metadata.hashes]
@@ -563,6 +603,7 @@ aiohttp = ["1e984191d1ec186881ffaed4581092ba04f7c61582a177b187d3a2f07ed9719e", "
 appdirs = ["9e5896d1372858f8dd3344faf4e5014d21849c756c8d5701f78f8a103b372d92", "d8b24664561d0d34ddfaec54636d502d7cea6e29c3eaf68f3df6180863e2166e"]
 aresponses = ["d1d6ef52b9a97142d106688cf9b112602ef3dc66f6368de8f91f47241d8cfc9c", "f62bcdd739612b6254dd552467b5897a81dcf785e4bb48463bf71e40df398580"]
 async-timeout = ["0c3c816a028d47f659d6ff5c745cb2acf1f966da1fe5c19c77a70282b25f4c5f", "4291ca197d287d274d0b6cb5d6f8f8f82d434ed288f962539ff18cc9012f9ea3"]
+asynctest = ["5da6118a7e6d6b54d83a8f7197769d046922a44d2a99c21382f0a6e4fadae676", "c27862842d15d83e6a34eb0b2866c323880eb3a75e4485b079ea11748fd77fac"]
 atomicwrites = ["03472c30eb2c5d1ba9227e4c2ca66ab8287fbfbbda3888aa93dc2e28fc6811b4", "75a9445bac02d8d058d5e1fe689654ba5a6556a1dfd8ce6ec55a0ed79866cfa6"]
 attrs = ["08a96c641c3a74e44eb59afb61a24f2cb9f4d7188748e76ba4bb5edfa3cb7d1c", "f7b7ce16570fe9965acd6d30101a28f62fb4a7f9e926b3bbc9b61f8b04247e72"]
 babel = ["af92e6106cb7c55286b25b38ad7695f8b4efb36a90ba483d7f7a6628c46158ab", "e86135ae101e31e2c8ec20a4e0c5220f4eed12487d5cf3f78be7e98d3a57fc28"]
@@ -603,6 +644,8 @@ pyparsing = ["20f995ecd72f2a1f4bf6b072b63b22e2eb457836601e76d6e5dfcd75436acc1f",
 pytest = ["15837d2880cb94821087bc07476892ea740696b20e90288fd6c19e44b435abdb", "b6cf7ad9064049ee486586b3a0ddd70dc5136c40e1147e7d286efd77ba66c5eb"]
 pytest-asyncio = ["9fac5100fd716cbecf6ef89233e8590a4ad61d729d1732e0a96b84182df1daaf", "d734718e25cfc32d2bf78d346e99d33724deeba774cc4afdf491530c6184b63b"]
 pytest-cov = ["cc6742d8bac45070217169f5f72ceee1e0e55b0221f54bcf24845972d3a47f2b", "cdbdef4f870408ebdbfeb44e63e07eb18bb4619fae852f6e760645fa36172626"]
+pytest-mock = ["b3514caac35fe3f05555923eabd9546abce11571cc2ddf7d8615959d04f2c89e", "ea502c3891599c26243a3a847ccf0b1d20556678c528f86c98e3cd6d40c5cf11"]
+pytest-mypy = ["3b7b56912d55439d5f447cc609f91caac7f74f0f1c89f1379d04f06bac777c32", "5a5338cecff17f005b181546a13e282761754b481225df37f33d37f86ac5b304"]
 pytz = ["1c557d7d0e871de1f5ccd5833f60fb2550652da6be2693c1e02300743d21500d", "b02c06db6cf09c12dd25137e563b31700d3b80fcc4ad23abb7a315f2789819be"]
 pyyaml = ["0113bc0ec2ad727182326b61326afa3d1d8280ae1122493553fd6f4397f33df9", "01adf0b6c6f61bd11af6e10ca52b7d4057dd0be0343eb9283c878cf3af56aee4", "5124373960b0b3f4aa7df1707e63e9f109b5263eca5976c66e08b1c552d4eaf8", "5ca4f10adbddae56d824b2c09668e91219bb178a1eee1faa56af6f99f11bf696", "7907be34ffa3c5a32b60b95f4d95ea25361c951383a894fec31be7252b2b6f34", "7ec9b2a4ed5cad025c2278a1e6a19c011c80a3caaac804fd2d329e9cc2c287c9", "87ae4c829bb25b9fe99cf71fbb2140c448f534e24c998cc60f39ae4f94396a73", "9de9919becc9cc2ff03637872a440195ac4241c80536632fffeb6a1e25a74299", "a5a85b10e450c66b49f98846937e8cfca1db3127a9d5d1e31ca45c3d0bef4c5b", "b0997827b4f6a7c286c01c5f60384d218dca4ed7d9efa945c3e1aa623d5709ae", "b631ef96d3222e62861443cc89d6563ba3eeb816eeb96b2629345ab795e53681", "bf47c0607522fdbca6c9e817a6e81b08491de50f3766a7a0e6a5be7905961b41", "f81025eddd0327c7d4cfe9b62cf33190e1e736cc6e97502b3ec425f574b3e7a8"]
 six = ["1f1b7d42e254082a9db6279deae68afb421ceba6158efa6131de7b3003ee93fd", "30f610279e8b2578cab6db20741130331735c781b56053c59c4076da27f06b66"]
diff --git a/pyproject.toml b/pyproject.toml
index 647b76b1..16ae3445 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -16,6 +16,7 @@ pydantic = "^1.1"
 Babel = "^2.7"
 pytest-cov = "^2.8"
 aiofiles = "^0.4.0"
+pytest-mock = "^1.11"
 
 [tool.poetry.dev-dependencies]
 black = {version = "^18.3-alpha.0", allows-prereleases = true}
@@ -30,6 +31,8 @@ mkdocs = "^1.0"
 mkdocs-material = "^4.4"
 pygments = "^2.4"
 pymdown-extensions = "^6.1"
+pytest-mypy = "^0.4.2"
+asynctest = "^0.13.0"
 
 [tool.black]
 line-length = 99
diff --git a/tests/test_api/__init__.py b/tests/test_api/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/test_api/test_client/__init__.py b/tests/test_api/test_client/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/test_api/test_methods/__init__.py b/tests/test_api/test_methods/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/test_api/test_session/__init__.py b/tests/test_api/test_session/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/test_api/test_session/test_api_server.py b/tests/test_api/test_session/test_api_server.py
new file mode 100644
index 00000000..2ff6fc26
--- /dev/null
+++ b/tests/test_api/test_session/test_api_server.py
@@ -0,0 +1,11 @@
+from aiogram.api.session.base import PRODUCTION
+
+
+class TestAPIServer:
+    def test_method_url(self):
+        method_url = PRODUCTION.api_url(token="TOKEN", method="apiMethod")
+        assert method_url == "https://api.telegram.org/botTOKEN/apiMethod"
+
+    def test_file_url(self):
+        file_url = PRODUCTION.file_url(token="TOKEN", path="path")
+        assert file_url == "https://api.telegram.org/file/botTOKEN/path"
diff --git a/tests/test_api/test_session/test_base_session.py b/tests/test_api/test_session/test_base_session.py
new file mode 100644
index 00000000..2d345059
--- /dev/null
+++ b/tests/test_api/test_session/test_base_session.py
@@ -0,0 +1,89 @@
+import datetime
+from unittest.mock import patch
+
+import pytest
+from asynctest import CoroutineMock
+
+from aiogram.api.session.base import BaseSession
+from aiogram.utils.mixins import DataMixin
+
+
+class TestBaseSession(DataMixin):
+    def setup(self):
+        self["__abstractmethods__"] = BaseSession.__abstractmethods__
+        BaseSession.__abstractmethods__ = set()
+
+    def teardown(self):
+        BaseSession.__abstractmethods__ = self["__abstractmethods__"]
+
+    def test_sync_close(self):
+        session = BaseSession()
+
+        with patch(
+            "aiogram.api.session.base.BaseSession.close", new=CoroutineMock()
+        ) as mocked_close:
+            session.__del__()
+            mocked_close.assert_called_once_with()
+
+    @pytest.mark.asyncio
+    async def test_async_close(self):
+        session = BaseSession()
+
+        with patch(
+            "aiogram.api.session.base.BaseSession.close", new=CoroutineMock()
+        ) as mocked_close:
+            session.__del__()
+            mocked_close.assert_called_once_with()
+
+    def test_prepare_value(self):
+        session = BaseSession()
+
+        now = datetime.datetime(
+            year=2019, month=11, day=15, hour=12, minute=42, second=15, microsecond=0
+        )
+
+        assert session.prepare_value("text") == "text"
+        assert session.prepare_value(["test"]) == '["test"]'
+        assert session.prepare_value({"test": "ok"}) == '{"test": "ok"}'
+        assert session.prepare_value(now) == "1573814535"
+        assert isinstance(session.prepare_value(datetime.timedelta(minutes=2)), str)
+        assert session.prepare_value(42) == "42"
+
+    def test_clean_json(self):
+        session = BaseSession()
+
+        cleaned_dict = session.clean_json({"key": "value", "null": None})
+        assert "key" in cleaned_dict
+        assert "null" not in cleaned_dict
+
+        cleaned_list = session.clean_json(["kaboom", 42, None])
+        assert len(cleaned_list) == 2
+        assert 42 in cleaned_list
+        assert None not in cleaned_list
+        assert cleaned_list[0] == "kaboom"
+
+    def test_clean_json_with_nested_json(self):
+        session = BaseSession()
+
+        cleaned = session.clean_json(
+            {
+                "key": "value",
+                "null": None,
+                "nested_list": ["kaboom", 42, None],
+                "nested_dict": {"key": "value", "null": None},
+            }
+        )
+
+        assert len(cleaned) == 3
+        assert "null" not in cleaned
+
+        assert isinstance(cleaned["nested_list"], list)
+        assert cleaned["nested_list"] == ["kaboom", 42]
+
+        assert isinstance(cleaned["nested_dict"], dict)
+        assert cleaned["nested_dict"] == {"key": "value"}
+
+    def test_clean_json_not_json(self):
+        session = BaseSession()
+
+        assert session.clean_json(42) == 42
diff --git a/tests/test_api/test_types/__init__.py b/tests/test_api/test_types/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/test_utils/test_helper.py b/tests/test_utils/test_helper.py
index 5477917e..ae07ce97 100644
--- a/tests/test_utils/test_helper.py
+++ b/tests/test_utils/test_helper.py
@@ -1,4 +1,5 @@
 import pytest
+
 from aiogram.utils.helper import Helper, HelperMode, Item, ListItem, OrderedHelper
 
 
diff --git a/tests/test_utils/test_mixins.py b/tests/test_utils/test_mixins.py
index 720bd38e..606d6aaa 100644
--- a/tests/test_utils/test_mixins.py
+++ b/tests/test_utils/test_mixins.py
@@ -1,4 +1,5 @@
 import pytest
+
 from aiogram.utils.mixins import ContextInstanceMixin, DataMixin
 
 
@@ -47,5 +48,7 @@ class TestContextInstanceMixin:
     def test_set_wrong_type(self):
         obj = ContextObject()
 
-        with pytest.raises(TypeError, match=r"Value should be instance of 'ContextObject' not '.+'"):
+        with pytest.raises(
+            TypeError, match=r"Value should be instance of 'ContextObject' not '.+'"
+        ):
             obj.set_current(42)