diff --git a/fin_defs/__init__.py b/fin_defs/__init__.py
index 12228dd..1217ef1 100644
--- a/fin_defs/__init__.py
+++ b/fin_defs/__init__.py
@@ -205,7 +205,7 @@ class Asset:
 @enforce_typing.enforce_types
 @dataclasses.dataclass(frozen=True)
 class UnknownAsset(Asset):
-    pass
+    """An asset that does not exist."""
 
     @abc.abstractmethod
     def raw_short_name(self) -> str:
@@ -215,12 +215,14 @@ class UnknownAsset(Asset):
 @enforce_typing.enforce_types
 @dataclasses.dataclass(frozen=True)
 class Currency(Asset):
-    pass
+    """Either a Fiat or a Crypto Currency."""
 
 
 @enforce_typing.enforce_types
 @dataclasses.dataclass(frozen=True, eq=True, order=True)
 class FiatCurrency(Currency):
+    """Fiat Currency."""
+
     iso_code: str
 
     def __post_init__(self) -> None:
@@ -253,6 +255,8 @@ FiatCurrency.JPY = FiatCurrency('JPY')
 @enforce_typing.enforce_types
 @dataclasses.dataclass(frozen=True, eq=True)
 class CryptoCurrency(Currency):
+    """Crypto Currency."""
+
     ccxt_symbol: str
     coingecko_id: str | None = None
 
diff --git a/test/test_asset_amount.py b/test/test_asset_amount.py
index c10982a..5c24961 100644
--- a/test/test_asset_amount.py
+++ b/test/test_asset_amount.py
@@ -82,7 +82,8 @@ def test_add_wrong_type():
 
 def test_add_wrong_asset():
     with pytest.raises(
-        ValueError, match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
+        ValueError,
+        match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
     ):
         assert USD_20 + DKK_21
 
@@ -94,7 +95,8 @@ def test_sub_wrong_type():
 
 def test_sub_wrong_asset():
     with pytest.raises(
-        ValueError, match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
+        ValueError,
+        match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
     ):
         assert USD_20 - DKK_21
 
@@ -106,13 +108,15 @@ def test_cmp_wrong_type():
 
 def test_cmp_wrong_asset():
     with pytest.raises(
-        ValueError, match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
+        ValueError,
+        match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
     ):
         assert USD_20 < DKK_21
 
 
 def test_div_wrong_asset():
     with pytest.raises(
-        ValueError, match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
+        ValueError,
+        match='AssetAmount must have same asset, but: fiat:USD != fiat:DKK',
     ):
         assert USD_21 / DKK_21
diff --git a/test/test_currency.py b/test/test_currency.py
index e941eae..6ce802e 100644
--- a/test/test_currency.py
+++ b/test/test_currency.py
@@ -1,4 +1,3 @@
-
 import fin_defs
 
 
diff --git a/test/test_exchange_rate_sample.py b/test/test_exchange_rate_sample.py
index b3b43b8..81fbed8 100644
--- a/test/test_exchange_rate_sample.py
+++ b/test/test_exchange_rate_sample.py
@@ -1,14 +1,15 @@
-
 import datetime
 from decimal import Decimal
 
 import fin_defs
 
-NOW = datetime.datetime.now()
+NOW = datetime.datetime.now(tz=datetime.UTC)
 THEN = NOW - datetime.timedelta(days=1)
 
 SAMPLE_AVERAGE = fin_defs.ExchangeRateSample(
-    NOW, (fin_defs.FiatCurrency.DKK, fin_defs.FiatCurrency.USD), _average=Decimal(1),
+    NOW,
+    (fin_defs.FiatCurrency.DKK, fin_defs.FiatCurrency.USD),
+    _average=Decimal(1),
 )
 
 SAMPLE_HIGH_LOW = fin_defs.ExchangeRateSample(
diff --git a/test/test_ids.py b/test/test_ids.py
index 8872de0..ff67351 100644
--- a/test/test_ids.py
+++ b/test/test_ids.py
@@ -29,7 +29,8 @@ ASSETS_POLYGON_PRESERVES_FULL_ID = frozenset(
     a
     for a in ASSETS
     if not isinstance(
-        a, fin_defs.CryptoCurrency | fin_defs.Commodity | fin_defs.UnknownAsset,
+        a,
+        fin_defs.CryptoCurrency | fin_defs.Commodity | fin_defs.UnknownAsset,
     )
 ) - frozenset([NVO])