diff --git a/src/marketdata/exceptions.py b/src/marketdata/exceptions.py
index 40fa336..c2225ec 100644
--- a/src/marketdata/exceptions.py
+++ b/src/marketdata/exceptions.py
@@ -28,9 +28,9 @@
 from datetime import datetime
 
 from httpx import Request, Response
-from pytz import timezone
 
 from marketdata.internal_settings import (
+    _DEFAULT_TIMEZONE,
     HEADER_AUTHORIZED_IP,
     HEADER_REQUEST_ID,
     read_header,
@@ -78,8 +78,17 @@ def format_timestamp(cls, timestamp: datetime) -> str:
 
     @classmethod
     def _coerce_timestamp(cls, timestamp: datetime | str | None) -> str:
+        """The exception's timestamp as text.
+
+        Args:
+            timestamp: A datetime, rendered by ``format_timestamp``; text, kept
+                as it is; or ``None`` for the current time in US/Eastern.
+
+        Returns:
+            The timestamp as ``YYYY-MM-DD HH:MM:SS``, or the text given.
+        """
         if timestamp is None:
-            return cls.format_timestamp(datetime.now(timezone("US/Eastern")))
+            return cls.format_timestamp(datetime.now(_DEFAULT_TIMEZONE))
         if isinstance(timestamp, datetime):
             return cls.format_timestamp(timestamp)
         return timestamp
diff --git a/src/marketdata/internal_settings.py b/src/marketdata/internal_settings.py
index bb76f6b..7fc3945 100644
--- a/src/marketdata/internal_settings.py
+++ b/src/marketdata/internal_settings.py
@@ -1,6 +1,7 @@
 import datetime
 
 import httpx
+import pytz
 from httpx import Response
 
 
@@ -32,6 +33,9 @@ class NoTokenValueType:
 # than that is a value the SDK could not read rather than a window that closed
 # (#42).
 MAX_CREDIT_WINDOW_SECONDS = 24 * 60 * 60
+# The zone of every timestamp the SDK renders. It is kept here, not in utils,
+# because utils imports exceptions, which needs it too.
+_DEFAULT_TIMEZONE = pytz.timezone("US/Eastern")
 
 # The API's IP headers (#44). httpx matches header names case-insensitively,
 # so the lowercase spelling reads them whatever case the API sends. The legacy
diff --git a/src/marketdata/output_handlers/pandas.py b/src/marketdata/output_handlers/pandas.py
index 4409c42..d8cd8c7 100644
--- a/src/marketdata/output_handlers/pandas.py
+++ b/src/marketdata/output_handlers/pandas.py
@@ -1,8 +1,8 @@
 import numpy as np
 import pandas as pd
-import pytz
 
 from marketdata.input_types.base import DateFormat
+from marketdata.internal_settings import _DEFAULT_TIMEZONE
 from marketdata.output_handlers.base import BaseOutputHandler
 
 _DTYPES = {float: "float64", int: "int64", bool: "bool", str: "object"}
@@ -50,12 +50,22 @@ def _convert_timestamp_columns(
         date_columns: list[str],
         date_format: DateFormat | None,
     ) -> pd.DataFrame:
-        """Convert date/time columns to timezone-aware datetime objects."""
+        """Convert the date columns to US/Eastern datetimes.
+
+        Args:
+            df: The result, with its columns cast.
+            date_columns: The columns to convert.
+            date_format: The request's date format. ``None`` reads the values
+                as Unix seconds, and ``DateFormat.UNIX`` leaves them as numbers.
+
+        Returns:
+            The same frame. A column that cannot be converted keeps its values.
+        """
         if date_format == DateFormat.UNIX:
             return df
 
         format_to_use = date_format or DateFormat.UNIX
-        default_tz = pytz.timezone("US/Eastern")
+        default_tz = _DEFAULT_TIMEZONE
 
         for col in df.columns:
             if col not in date_columns:
diff --git a/src/marketdata/output_handlers/polars.py b/src/marketdata/output_handlers/polars.py
index 5fb466e..5778eaf 100644
--- a/src/marketdata/output_handlers/polars.py
+++ b/src/marketdata/output_handlers/polars.py
@@ -1,7 +1,7 @@
 import polars as pl
-import pytz
 
 from marketdata.input_types.base import DateFormat
+from marketdata.internal_settings import _DEFAULT_TIMEZONE
 from marketdata.output_handlers.base import BaseOutputHandler
 
 _DTYPES = {float: pl.Float64, int: pl.Int64, bool: pl.Boolean, str: pl.String}
@@ -40,12 +40,23 @@ def _convert_timestamp_columns(
         date_columns: list[str],
         date_format: DateFormat | None,
     ) -> pl.DataFrame:
-        """Convert date/time columns to timezone-aware datetime objects."""
+        """Convert the date columns to US/Eastern datetimes.
+
+        Args:
+            df: The result, with its columns cast.
+            date_columns: The columns to convert.
+            date_format: The request's date format. ``None`` reads the values
+                as Unix seconds, and ``DateFormat.UNIX`` leaves them as numbers.
+
+        Returns:
+            The frame with its date columns converted. A column that cannot be
+            converted keeps its values.
+        """
         if date_format == DateFormat.UNIX:
             return df
 
         format_to_use = date_format or DateFormat.UNIX
-        default_tz = pytz.timezone("US/Eastern").zone
+        default_tz = _DEFAULT_TIMEZONE.zone
 
         for col in df.columns:
             if col not in date_columns:
diff --git a/src/marketdata/types.py b/src/marketdata/types.py
index b2565ea..74ea8fd 100644
--- a/src/marketdata/types.py
+++ b/src/marketdata/types.py
@@ -3,7 +3,7 @@
 
 from pytz.exceptions import InvalidTimeError
 
-from marketdata.utils import DEFAULT_TIMEZONE
+from marketdata.internal_settings import _DEFAULT_TIMEZONE
 
 
 @dataclass
@@ -36,7 +36,7 @@ def __post_init__(self):
                 raise refused
             try:
                 reset_time = datetime.datetime.fromtimestamp(
-                    float(reset_time), tz=DEFAULT_TIMEZONE
+                    float(reset_time), tz=_DEFAULT_TIMEZONE
                 )
             except (TypeError, ValueError, OverflowError, OSError):
                 raise refused from None
@@ -44,7 +44,7 @@ def __post_init__(self):
             try:
                 # `is_dst=None`: a repeated or skipped wall time names no single
                 # instant, and picking one would move the reset by an hour.
-                reset_time = DEFAULT_TIMEZONE.localize(reset_time, is_dst=None)
+                reset_time = _DEFAULT_TIMEZONE.localize(reset_time, is_dst=None)
             except InvalidTimeError as exc:
                 raise ValueError(
                     f"{reset_time.isoformat()} is not one US/Eastern time: a "
diff --git a/src/marketdata/utils.py b/src/marketdata/utils.py
index 180f209..151441f 100644
--- a/src/marketdata/utils.py
+++ b/src/marketdata/utils.py
@@ -7,16 +7,13 @@
 from typing import Any, Iterator, NoReturn
 from urllib.parse import quote
 
-import pytz
 from httpx import Response
 
 from marketdata.exceptions import ParseError
-from marketdata.internal_settings import VALID_STATUS_CODES
+from marketdata.internal_settings import _DEFAULT_TIMEZONE, VALID_STATUS_CODES
 
-# Every timestamp the SDK renders is US/Eastern. One constant, so the zone a
-# naive value is read in and the zone it is rendered in agree by construction
-# rather than because two literals happen to match.
-DEFAULT_TIMEZONE = pytz.timezone("US/Eastern")
+# Assigned, not imported: an imported name is not part of the public surface.
+DEFAULT_TIMEZONE = _DEFAULT_TIMEZONE
 
 
 def _exact_number(text: str) -> Decimal:
diff --git a/src/tests/test_default_timezone.py b/src/tests/test_default_timezone.py
new file mode 100644
index 0000000..ec83775
--- /dev/null
+++ b/src/tests/test_default_timezone.py
@@ -0,0 +1,111 @@
+"""Every timestamp the SDK renders takes its zone from one constant."""
+
+import datetime
+import time
+from unittest.mock import patch
+
+import pytest
+import pytz
+
+from marketdata import exceptions, get_meta, types, utils
+from marketdata.exceptions import BadRequestError
+from marketdata.input_types.base import OutputFormat
+from marketdata.output_handlers import pandas as pandas_handler
+from marketdata.output_handlers import polars as polars_handler
+from src.tests.conftest import use_real_header_extraction
+
+QUOTES_URL = "https://api.marketdata.app/v1/stocks/quotes/"
+UPDATED = 1765552906
+TOKYO = pytz.timezone("Asia/Tokyo")
+STAMP = "%Y-%m-%d %H:%M:%S"
+
+
+@pytest.fixture
+def tokyo(monkeypatch):
+    """Point every module that renders a timestamp at Asia/Tokyo, a zone with
+    no daylight saving time and 13 or 14 hours away from US/Eastern.
+
+    Returns:
+        The Asia/Tokyo zone.
+    """
+    for module, name in (
+        (utils, "DEFAULT_TIMEZONE"),
+        (types, "_DEFAULT_TIMEZONE"),
+        (exceptions, "_DEFAULT_TIMEZONE"),
+        (pandas_handler, "_DEFAULT_TIMEZONE"),
+        (polars_handler, "_DEFAULT_TIMEZONE"),
+    ):
+        monkeypatch.setattr(module, name, TOKYO)
+    return TOKYO
+
+
+def test_utils_default_timezone_is_us_eastern():
+    """`marketdata.utils.DEFAULT_TIMEZONE` is the US/Eastern zone."""
+    assert utils.DEFAULT_TIMEZONE is pytz.timezone("US/Eastern")
+
+
+def test_internal_dates_and_the_credit_reset_follow_the_constant(
+    tokyo, load_json, respx_mock, client
+):
+    """An INTERNAL result's dates and the credit reset read from its headers
+    are in the zone the constant holds."""
+    use_real_header_extraction(client)
+    reset = int(time.time()) + 60
+    respx_mock.get(QUOTES_URL).respond(
+        json=load_json("stocks_quotes_response_200"),
+        headers={
+            "x-api-ratelimit-limit": "100",
+            "x-api-ratelimit-remaining": "99",
+            "x-api-ratelimit-reset": str(reset),
+            "x-api-ratelimit-consumed": "1",
+        },
+        status_code=200,
+    )
+
+    quotes = client.stocks.quotes(
+        symbols=["AAPL", "MSFT"], output_format=OutputFormat.INTERNAL
+    )
+    reset_time = get_meta(quotes).rate_limits.reset_time
+
+    assert quotes[0].updated == datetime.datetime.fromtimestamp(UPDATED, tz=tokyo)
+    assert str(quotes[0].updated.tzinfo) == "Asia/Tokyo"
+    assert reset_time.timestamp() == reset
+    assert str(reset_time.tzinfo) == "Asia/Tokyo"
+
+
+@pytest.mark.parametrize("library", ["pandas", "polars"])
+def test_dataframe_dates_follow_the_constant(
+    tokyo, load_json, respx_mock, client, library
+):
+    """A DataFrame's date column is in the zone the constant holds.
+
+    Args:
+        library: The DataFrame library the client builds the result with.
+    """
+    respx_mock.get(QUOTES_URL).respond(
+        json=load_json("stocks_quotes_response_200"), status_code=200
+    )
+
+    with patch("marketdata.output_handlers.DATAFRAME_HANDLERS_PRIORITY", [library]):
+        frame = client.stocks.quotes(
+            symbols=["AAPL", "MSFT"], output_format=OutputFormat.DATAFRAME
+        )
+    updated = frame["updated"].to_list()[0]
+
+    assert updated == datetime.datetime.fromtimestamp(UPDATED, tz=tokyo)
+    assert str(updated.tzinfo) == "Asia/Tokyo"
+
+
+def test_an_error_timestamp_follows_the_constant(tokyo, respx_mock, client):
+    """An error the API answered is stamped with the time in the zone the
+    constant holds."""
+    respx_mock.get(QUOTES_URL).respond(
+        json={"errmsg": "Invalid symbol"}, status_code=400
+    )
+
+    before = datetime.datetime.now(tokyo).strftime(STAMP)
+    with pytest.raises(BadRequestError) as caught:
+        client.stocks.quotes(symbols=["AAPL"], output_format=OutputFormat.INTERNAL)
+    after = datetime.datetime.now(tokyo).strftime(STAMP)
+
+    assert before <= caught.value.timestamp <= after