[SPARK-31940][SQL][DOCS] Document the default JVM time zone in to/fromJavaDate and legacy date formatters

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateFormatter.scala

@@ -100,6 +100,17 @@ trait LegacyDateFormatter extends DateFormatter {
   }
 }
 
+/**
+ * The legacy formatter is based on Apache Commons FastDateFormat. The formatter uses the default
+ * JVM time zone intentionally for compatibility with Spark 2.4 and earlier versions.
+ *
+ * Note: Using of the default JVM time zone makes the formatter compatible with the legacy
+ *       `DateTimeUtils` methods `toJavaDate` and `fromJavaDate` that are based on the default
+ *       JVM time zone too.
+ *
+ * @param pattern `java.text.SimpleDateFormat` compatible pattern.
+ * @param locale The locale overrides the system locale and is used in parsing/formatting.
+ */
 class LegacyFastDateFormatter(pattern: String, locale: Locale) extends LegacyDateFormatter {
   @transient
   private lazy val fdf = FastDateFormat.getInstance(pattern, locale)
@@ -108,6 +119,22 @@ class LegacyFastDateFormatter(pattern: String, locale: Locale) extends LegacyDat
   override def validatePatternString(): Unit = fdf
 }
 
+// scalastyle:off line.size.limit
+/**
+ * The legacy formatter is based on `java.text.SimpleDateFormat`. The formatter uses the default
+ * JVM time zone intentionally for compatibility with Spark 2.4 and earlier versions.
+ *
+ * Note: Using of the default JVM time zone makes the formatter compatible with the legacy
+ *       `DateTimeUtils` methods `toJavaDate` and `fromJavaDate` that are based on the default
+ *       JVM time zone too.
+ *
+ * @param pattern The pattern describing the date and time format.
+ *                See <a href="https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html">
+ *                Date and Time Patterns</a>
+ * @param locale  The locale whose date format symbols should be used. It overrides the system
+ *                locale in parsing/formatting.
+ */
+// scalastyle:on line.size.limit
 class LegacySimpleDateFormatter(pattern: String, locale: Locale) extends LegacyDateFormatter {
   @transient
   private lazy val sdf = new SimpleDateFormat(pattern, locale)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala

@@ -88,19 +88,20 @@ object DateTimeUtils {
   }
 
   /**
-   * Converts an instance of `java.sql.Date` to a number of days since the epoch
-   * 1970-01-01 via extracting date fields `year`, `month`, `days` from the input,
-   * creating a local date in Proleptic Gregorian calendar from the fields, and
-   * getting the number of days from the resulted local date.
+   * Converts a local date at the default JVM time zone to the number of days since 1970-01-01
+   * in the hybrid calendar (Julian + Gregorian) by discarding the time part. The resulted days are
+   * rebased from the hybrid to Proleptic Gregorian calendar. The days rebasing is performed via
+   * UTC time zone for simplicity because the difference between two calendars is the same in
+   * any given time zone and UTC time zone.
    *
-   * This approach was taken to have the same local date as the triple of `year`,
-   * `month`, `day` in the original hybrid calendar used by `java.sql.Date` and
-   * Proleptic Gregorian calendar used by Spark since version 3.0.0, see SPARK-26651.
+   * Note: The date is shifted by the offset of the default JVM time zone for backward compatibility
+   *       with Spark 2.4 and earlier versions. The goal of the shift is to get a local date derived
+   *       from the number of days that has the same date fields (year, month, day) as the original
+   *       `date` at the default JVM time zone.
    *
-   * @param date It represents a specific instant in time based on
-   *             the hybrid calendar which combines Julian and
-   *             Gregorian calendars.
-   * @return The number of days since epoch from java.sql.Date.
+   * @param date It represents a specific instant in time based on the hybrid calendar which
+   *             combines Julian and Gregorian calendars.
+   * @return The number of days since the epoch in Proleptic Gregorian calendar.
    */
   def fromJavaDate(date: Date): SQLDate = {
     val millisUtc = date.getTime
@@ -110,17 +111,18 @@ object DateTimeUtils {
   }
 
   /**
-   * The opposite to `fromJavaDate` method which converts a number of days to an
-   * instance of `java.sql.Date`. It builds a local date in Proleptic Gregorian
-   * calendar, extracts date fields `year`, `month`, `day`, and creates a local
-   * date in the hybrid calendar (Julian + Gregorian calendars) from the fields.
+   * Converts days since the epoch 1970-01-01 in Proleptic Gregorian calendar to a local date
+   * at the default JVM time zone in the hybrid calendar (Julian + Gregorian). It rebases the given
+   * days from Proleptic Gregorian to the hybrid calendar at UTC time zone for simplicity because
+   * the difference between two calendars doesn't depend on any time zone. The result is shifted
+   * by the time zone offset in wall clock to have the same date fields (year, month, day)
+   * at the default JVM time zone as the input `daysSinceEpoch` in Proleptic Gregorian calendar.
    *
-   * The purpose of the conversion is to have the same local date as the triple
-   * of `year`, `month`, `day` in the original Proleptic Gregorian calendar and
-   * in the target calender.
+   * Note: The date is shifted by the offset of the default JVM time zone for backward compatibility
+   *       with Spark 2.4 and earlier versions.
    *
-   * @param daysSinceEpoch The number of days since 1970-01-01.
-   * @return A `java.sql.Date` from number of days since epoch.
+   * @param daysSinceEpoch The number of days since 1970-01-01 in Proleptic Gregorian calendar.
+   * @return A local date in the hybrid calendar as `java.sql.Date` from number of days since epoch.
    */
   def toJavaDate(daysSinceEpoch: SQLDate): Date = {
     val rebasedDays = rebaseGregorianToJulianDays(daysSinceEpoch)