GH-1293: Use floor division when splitting epoch millis into day and time (#1294)

## What's Changed

`DateTimeUtils.getTimestampValue(long)` used `/` and `%` to split epoch
milliseconds into an epoch day and a time within that day. These
operators round toward zero. For negative values that were not exactly
midnight, the existing code fixed the remainder but not the epoch day.
The two parts then referred to different days, so the timestamp came
back one day late.

For example, `-618102000000` ms is 1950-06-01 01:00:00 UTC. The old
division produced epoch day `-7153`, which is 1950-06-02, while the
remainder was 01:00. The method returned 1950-06-02 01:00:00.

This affects DATE values before 1970 when
`ArrowFlightJdbcDateVectorAccessor.getDate(Calendar)` applies a non-zero
calendar offset. The offset moves the value away from midnight and
exposes the division bug.

Closes #1293.

Author: AbhishekPathania

flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/utils/DateTimeUtils.java

@@ -55,15 +55,12 @@ public static long applyCalendarOffset(long milliseconds, Calendar calendar) {
    * @return a {@link Timestamp} object representing the given Epoch millis
    */
   public static Timestamp getTimestampValue(long millisWithCalendar) {
-    long milliseconds = millisWithCalendar;
-    if (milliseconds < 0) {
-      // LocalTime#ofNanoDay only accepts positive values
-      milliseconds -= ((milliseconds / MILLIS_PER_DAY) - 1) * MILLIS_PER_DAY;
-    }
-
+    // Millis are negative before 1970, where only floor semantics keep the epoch day
+    // and the time-of-day remainder on the same day (and the remainder non-negative).
     return Timestamp.valueOf(
         LocalDateTime.of(
-            LocalDate.ofEpochDay(millisWithCalendar / MILLIS_PER_DAY),
-            LocalTime.ofNanoOfDay(TimeUnit.MILLISECONDS.toNanos(milliseconds % MILLIS_PER_DAY))));
+            LocalDate.ofEpochDay(Math.floorDiv(millisWithCalendar, MILLIS_PER_DAY)),
+            LocalTime.ofNanoOfDay(
+                TimeUnit.MILLISECONDS.toNanos(Math.floorMod(millisWithCalendar, MILLIS_PER_DAY)))));
   }
 }

flight/flight-sql-jdbc-core/src/test/java/org/apache/arrow/driver/jdbc/utils/DateTimeUtilsTest.java

@@ -95,4 +95,15 @@ public void testShouldGetTimestampNegative() {
 
     assertThat(expected, is(actual));
   }
+
+  @Test
+  public void testShouldGetTimestampNegativeNotAlignedToDay() {
+    final long epochMilli = negativeEpochMilli + 3600000L; // 1950-06-01 01:00:00 UTC
+    final Instant instant = Instant.ofEpochMilli(epochMilli);
+
+    final Timestamp expected = Timestamp.from(instant);
+    final Timestamp actual = DateTimeUtils.getTimestampValue(epochMilli);
+
+    assertThat(expected, is(actual));
+  }
 }