From 9029128efa1bdcd1fd1b2afa9c4762871ccf03ac Mon Sep 17 00:00:00 2001
From: Stefan Fochler <stefan.fochler@almato.com>
Date: Thu, 12 Feb 2026 14:00:59 +0100
Subject: [PATCH] Fix encode_param for DateTime

---
 lib/ch/query.ex        | 32 +++++++++++++++++++++++++------
 test/ch/query_test.exs | 43 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 6 deletions(-)

diff --git a/lib/ch/query.ex b/lib/ch/query.ex
index 37e0596..6f51c4f 100644
--- a/lib/ch/query.ex
+++ b/lib/ch/query.ex
@@ -347,14 +347,34 @@ defimpl DBConnection.Query, for: Ch.Query do
         seconds = div(unix, size)
         fractional = rem(unix, size)
 
-        IO.iodata_to_binary([
-          Integer.to_string(seconds),
-          ?.,
-          String.pad_leading(Integer.to_string(fractional), precision, "0")
-        ])
+        # Manually add minus sign if fractional is < 0 and seconds isn't already negative.
+        sign = if fractional < 0 and seconds >= 0, do: [?-], else: []
+
+        fractional = abs(fractional)
+
+        IO.iodata_to_binary(
+          sign ++
+            [
+              Integer.to_string(seconds),
+              ?.,
+              String.pad_leading(Integer.to_string(fractional), precision, "0")
+            ]
+        )
 
       _ ->
-        dt |> DateTime.to_unix(:second) |> Integer.to_string()
+        # Padding needed for small values: https://github.com/ClickHouse/ClickHouse/issues/64708
+        dt = dt |> DateTime.to_unix(:second)
+
+        sign = if dt < 0, do: [?-], else: []
+
+        IO.iodata_to_binary(
+          sign ++
+            [
+              abs(dt)
+              |> Integer.to_string()
+              |> String.pad_leading(5, "0")
+            ]
+        )
     end
   end
 
diff --git a/test/ch/query_test.exs b/test/ch/query_test.exs
index 056cb0c..3d7532a 100644
--- a/test/ch/query_test.exs
+++ b/test/ch/query_test.exs
@@ -454,6 +454,49 @@ defmodule Ch.QueryTest do
       # assert [[[1, nil, 3]]] = Ch.query!(conn, "SELECT {$0:Array(integer)}", [[1, nil, 3]], query_options).rows
     end
 
+    test "encode datetimes close to unix epoch", %{conn: conn, query_options: query_options} do
+      assert [[~U[1970-01-01 00:00:00Z]]] ==
+               Ch.query!(
+                 conn,
+                 "SELECT {$0:DateTime('UTC')}",
+                 [~U[1970-01-01 00:00:00Z]],
+                 query_options
+               ).rows
+
+      assert [[~U[1970-01-01 00:00:00.001Z]]] ==
+               Ch.query!(
+                 conn,
+                 "SELECT {$0:DateTime64(3, 'UTC')}",
+                 [~U[1970-01-01 00:00:00.001Z]],
+                 query_options
+               ).rows
+
+      assert [[~U[1969-12-31 23:59:59Z]]] ==
+               Ch.query!(
+                 conn,
+                 "SELECT {$0:DateTime64(0, 'UTC')}",
+                 [~U[1969-12-31 23:59:59Z]],
+                 query_options
+               ).rows
+
+      # This is currently blocked on https://github.com/ClickHouse/ClickHouse/issues/96745
+      # assert [[~U[1969-12-31 23:59:59.500Z]]] ==
+      #          Ch.query!(
+      #            conn,
+      #            "SELECT {$0:DateTime64(3, 'UTC')}",
+      #            [~U[1969-12-31 23:59:59.500Z]],
+      #            query_options
+      #          ).rows
+
+      assert [[~U[1969-12-31 23:59:58.500Z]]] ==
+               Ch.query!(
+                 conn,
+                 "SELECT {$0:DateTime64(3, 'UTC')}",
+                 [~U[1969-12-31 23:59:58.500Z]],
+                 query_options
+               ).rows
+    end
+
     test "encode network types", %{conn: conn, query_options: query_options} do
       # TODO, or wrap in custom struct like in postgrex
       # assert [["127.0.0.1/32"]] =