[BugFix] Fix pre-1970 Parquet timestamp load corrupting sub-second DATETIME

be/src/formats/parquet/column_converter.cpp

@@ -912,6 +912,14 @@ Status Int64ToDateTimeConverter::convert(const Column* src, Column* dst) {
             if (!src_null_data[i]) {
                 int64_t seconds = src_data[i] / _second_mask;
                 int64_t nanoseconds = (src_data[i] % _second_mask) * _scale_to_nano_factor;
+                // Truncating division leaves a negative sub-second remainder for a pre-1970 tick;
+                // borrow a whole second so nanoseconds stays in [0, NANOSECS_PER_SEC), matching the
+                // floor split the FE boundary computation uses. Without this, of_epoch_second packs
+                // a negative microsecond into the timestamp and corrupts the value.
+                if (nanoseconds < 0) {
+                    seconds -= 1;
+                    nanoseconds += NANOSECS_PER_SEC;
+                }
 
                 if constexpr (UTC_TO_TZ) {
                     int offset = _offset;

be/test/formats/parquet/column_converter_test.cpp

@@ -766,4 +766,25 @@ TEST_F(ColumnConverterTest, Int64_2_Timestamp) {
         }
     }
 }
+
+// A pre-1970 (negative epoch tick) timestamp with a nonzero sub-second component must decode to the
+// correct wall clock. C++ truncating division splits a negative tick into a too-high second and a
+// negative sub-second; without a floor-borrow that negative sub-second corrupts the packed DATETIME.
+TEST_F(ColumnConverterTest, Int64PreEpochTimestampSubSecond) {
+    const std::string file_path =
+            "./be/test/formats/parquet/test_data/column_converter/int64_timestamp_pre_epoch.parquet";
+    const size_t expected_rows = 5;
+    const std::string expected_value = "[1969-12-31 23:59:59.500000]";
+
+    {
+        const std::string col_name = "timestamp_millis";
+        const TypeDescriptor col_type = TypeDescriptor::from_logical_type(LogicalType::TYPE_DATETIME);
+        check(file_path, col_type, col_name, expected_value, expected_rows);
+    }
+    {
+        const std::string col_name = "timestamp_micros";
+        const TypeDescriptor col_type = TypeDescriptor::from_logical_type(LogicalType::TYPE_DATETIME);
+        check(file_path, col_type, col_name, expected_value, expected_rows);
+    }
+}
 } // namespace starrocks::parquet