diff --git a/sdks/python/pmxt/_hosted_mappers.py b/sdks/python/pmxt/_hosted_mappers.py
index 30f0adab..335d815f 100644
--- a/sdks/python/pmxt/_hosted_mappers.py
+++ b/sdks/python/pmxt/_hosted_mappers.py
@@ -8,7 +8,7 @@
 from __future__ import annotations
 
 import dataclasses as _dc
-from datetime import datetime, timezone
+from datetime import date, datetime, timezone
 from decimal import Decimal
 from typing import Any, Mapping, TypeVar
 
@@ -362,3 +362,17 @@ def _ms_to_timestamp(value: Any) -> str | None:
     dt = datetime.fromtimestamp(int(value) / 1000, tz=timezone.utc)
     return dt.isoformat().replace("+00:00", "Z")
 
+
+def _format_datetime_utc(value: datetime | date) -> str:
+    """Serialize a datetime like JavaScript ``Date.toISOString()``.
+
+    Naive values are treated as UTC, aware values are converted to UTC, and
+    bare dates count as UTC midnight.  The result always has millisecond
+    precision and a ``Z`` suffix, e.g. ``2026-01-01T00:00:00.000Z``.
+    """
+    dt = value if isinstance(value, datetime) else datetime(value.year, value.month, value.day)
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=timezone.utc)
+    else:
+        dt = dt.astimezone(timezone.utc)
+    return dt.isoformat(timespec="milliseconds").replace("+00:00", "Z")
diff --git a/sdks/python/pmxt/client.py b/sdks/python/pmxt/client.py
index 96e1e56d..4ed44972 100644
--- a/sdks/python/pmxt/client.py
+++ b/sdks/python/pmxt/client.py
@@ -80,6 +80,7 @@
     resolve_wallet_address,
 )
 from ._hosted_mappers import (
+    _format_datetime_utc,
     balance_from_v0,
     built_order_from_v0,
     order_from_v0,
@@ -2372,9 +2373,9 @@ def fetch_ohlcv(
             if resolution:
                 params_dict["resolution"] = resolution
             if start:
-                params_dict["start"] = start.isoformat()
+                params_dict["start"] = _format_datetime_utc(start)
             if end:
-                params_dict["end"] = end.isoformat()
+                params_dict["end"] = _format_datetime_utc(end)
             if limit:
                 params_dict["limit"] = limit
 
diff --git a/sdks/python/pmxt/router.py b/sdks/python/pmxt/router.py
index 00e0071c..09a0e95e 100644
--- a/sdks/python/pmxt/router.py
+++ b/sdks/python/pmxt/router.py
@@ -11,6 +11,7 @@
 from typing import Any, Dict, List, Optional, Union
 
 from .client import Exchange, _convert_market, _convert_event
+from ._hosted_mappers import _format_datetime_utc
 from .models import (
     MatchResult,
     EventMatchResult,
@@ -66,7 +67,7 @@ def _parse_match_result(raw: Dict[str, Any]) -> MatchResult:
 
 def _normalize_query_value(value: Any) -> Any:
     if isinstance(value, (datetime, date)):
-        return value.isoformat()
+        return _format_datetime_utc(value)
     return value
 
 
diff --git a/sdks/python/tests/test_datetime_serialization.py b/sdks/python/tests/test_datetime_serialization.py
new file mode 100644
index 00000000..28cead0e
--- /dev/null
+++ b/sdks/python/tests/test_datetime_serialization.py
@@ -0,0 +1,135 @@
+"""Unit tests for datetime parameter serialization shared by client and router.
+
+The Python SDK must send the same wire values as the TypeScript SDK's
+``Date.toISOString()`` for the same wall-clock input (#2095, #2096): naive
+datetimes are treated as UTC, aware datetimes are converted to UTC, and the
+result always carries millisecond precision with a ``Z`` suffix.
+
+The transport layer is mocked so no server or network access happens.
+"""
+
+from __future__ import annotations
+
+import json
+from datetime import date, datetime, timedelta, timezone
+from typing import Any, Dict
+from urllib.parse import parse_qs, urlparse
+
+from pmxt._exchanges import Mock
+from pmxt.router import Router
+
+
+PMXT_API_KEY = "test_pmxt_key_xxx"
+BASE_URL = "https://api.example.test"
+
+
+class _FakeResponse:
+    def __init__(self, payload: Dict[str, Any]) -> None:
+        self.data = json.dumps(payload).encode("utf-8")
+
+    def read(self) -> None:  # mirrors urllib3 HTTPResponse.read()
+        return None
+
+
+# --------------------------------------------------------------------------- #
+# fetch_ohlcv (client.py)                                                     #
+# --------------------------------------------------------------------------- #
+
+
+def _capture_ohlcv_params(monkeypatch, **kwargs) -> Dict[str, Any]:
+    exchange = Mock(auto_start_server=False)
+    captured: Dict[str, Any] = {}
+
+    def fake_sidecar_read_request(method_name, query, args):
+        captured["params"] = args[1]
+        return {"success": True, "data": []}
+
+    monkeypatch.setattr(exchange, "_sidecar_read_request", fake_sidecar_read_request)
+    candles = exchange.fetch_ohlcv("outcome-123", resolution="1h", **kwargs)
+    assert candles == []
+    return captured["params"]
+
+
+def test_fetch_ohlcv_naive_datetimes_serialized_as_utc(monkeypatch):
+    params = _capture_ohlcv_params(
+        monkeypatch,
+        start=datetime(2026, 1, 1),
+        end=datetime(2026, 1, 31, 23, 59, 59),
+    )
+
+    assert params["start"] == "2026-01-01T00:00:00.000Z"
+    assert params["end"] == "2026-01-31T23:59:59.000Z"
+
+
+def test_fetch_ohlcv_aware_datetimes_converted_to_utc(monkeypatch):
+    pst = timezone(timedelta(hours=-8))
+    params = _capture_ohlcv_params(monkeypatch, start=datetime(2026, 1, 1, tzinfo=pst))
+
+    # Same instant as TS: new Date("2026-01-01T00:00:00-08:00").toISOString()
+    assert params["start"] == "2026-01-01T08:00:00.000Z"
+
+
+def test_fetch_ohlcv_microseconds_truncated_to_milliseconds(monkeypatch):
+    params = _capture_ohlcv_params(
+        monkeypatch, start=datetime(2026, 6, 15, 12, 30, 45, 123456)
+    )
+
+    assert params["start"] == "2026-06-15T12:30:45.123Z"
+
+
+# --------------------------------------------------------------------------- #
+# Router updatedSince (router.py _normalize_query_value)                       #
+# --------------------------------------------------------------------------- #
+
+
+def _captured_updated_since(monkeypatch, **kwargs) -> str:
+    router = Router(
+        pmxt_api_key=PMXT_API_KEY, base_url=BASE_URL, auto_start_server=False
+    )
+    calls = []
+
+    def fake_call_api(method=None, url=None, body=None, header_params=None, **kw):
+        calls.append({"method": method, "url": url})
+        return _FakeResponse({"data": []})
+
+    monkeypatch.setattr(router._api_client, "call_api", fake_call_api)
+    router.fetch_matched_event_clusters(**kwargs)
+
+    query = parse_qs(urlparse(calls[0]["url"]).query)
+    return query["updatedSince"][0]
+
+
+def test_router_updated_since_naive_datetime_serialized_as_utc(monkeypatch):
+    value = _captured_updated_since(
+        monkeypatch, updated_since=datetime(2026, 1, 2, 3, 4, 5)
+    )
+
+    assert value == "2026-01-02T03:04:05.000Z"
+
+
+def test_router_updated_since_aware_datetime_converted_to_utc(monkeypatch):
+    jst = timezone(timedelta(hours=9))
+    value = _captured_updated_since(
+        monkeypatch, updated_since=datetime(2026, 1, 2, 12, 0, 0, tzinfo=jst)
+    )
+
+    assert value == "2026-01-02T03:00:00.000Z"
+
+
+def test_router_updated_since_bare_date_is_utc_midnight(monkeypatch):
+    value = _captured_updated_since(monkeypatch, updated_since=date(2026, 1, 1))
+
+    assert value == "2026-01-01T00:00:00.000Z"
+
+
+def test_router_matches_typescript_date_toisostring_output(monkeypatch):
+    # router.ts sends new Date(Date.UTC(2026, 0, 1)).toISOString() for this input.
+    value = _captured_updated_since(monkeypatch, updated_since=datetime(2026, 1, 1))
+
+    assert value == "2026-01-01T00:00:00.000Z"
+
+
+def test_router_string_values_pass_through_unchanged(monkeypatch):
+    value = _captured_updated_since(monkeypatch, updated_since="2026-01-02T03:04:05Z")
+
+    assert value == "2026-01-02T03:04:05Z"