[SPARK-30865][SQL][SS] Refactor DateTimeUtils

### What changes were proposed in this pull request?

1. Move TimeZoneUTC and TimeZoneGMT to DateTimeTestUtils
2. Remove TimeZoneGMT
3. Use ZoneId.systemDefault() instead of defaultTimeZone().toZoneId
4. Alias SQLDate & SQLTimestamp to internal types of DateType and TimestampType
5. Avoid one `*` `DateTimeUtils`.`in fromJulianDay()`
6. Use toTotalMonths in `DateTimeUtils`.`subtractDates()`
7. Remove `julianCommonEraStart`, `timestampToString()`, `microsToEpochDays()`, `epochDaysToMicros()`, `instantToDays()` from `DateTimeUtils`.
8. Make splitDate() private.
9. Remove `def daysToMicros(days: Int): Long` and `def microsToDays(micros: Long): Int`.

### Why are the changes needed?

This simplifies the common code related to date-time operations, and should improve maintainability. In particular:

1. TimeZoneUTC and TimeZoneGMT are moved to DateTimeTestUtils because they are used only in tests
2. TimeZoneGMT can be removed because it is equal to TimeZoneUTC
3. After the PR #27494, Spark expressions and DateTimeUtils functions switched to ZoneId instead of TimeZone completely. `defaultTimeZone()` with `TimeZone` as return type is not needed anymore.
4. SQLDate and SQLTimestamp types can be explicitly aliased to internal types of DateType and and TimestampType instead of declaring this in a comment.
5. Avoid one `*` `DateTimeUtils`.`in fromJulianDay()`.
6. Use toTotalMonths in `DateTimeUtils`.`subtractDates()`.

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
By existing test suites

Closes #27617 from MaxGekk/move-time-zone-consts.

Lead-authored-by: Max Gekk <max.gekk@gmail.com>
Co-authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: MaxGekk

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala

@@ -303,7 +303,7 @@ abstract class CastBase extends UnaryExpression with TimeZoneAwareExpression wit
     case BinaryType => buildCast[Array[Byte]](_, UTF8String.fromBytes)
     case DateType => buildCast[Int](_, d => UTF8String.fromString(dateFormatter.format(d)))
     case TimestampType => buildCast[Long](_,
-      t => UTF8String.fromString(DateTimeUtils.timestampToString(timestampFormatter, t)))
+      t => UTF8String.fromString(timestampFormatter.format(t)))
     case ArrayType(et, _) =>
       buildCast[ArrayData](_, array => {
         val builder = new UTF8StringBuilder
@@ -443,7 +443,7 @@ abstract class CastBase extends UnaryExpression with TimeZoneAwareExpression wit
     case ByteType =>
       buildCast[Byte](_, b => longToTimestamp(b.toLong))
     case DateType =>
-      buildCast[Int](_, d => epochDaysToMicros(d, zoneId))
+      buildCast[Int](_, d => daysToMicros(d, zoneId))
     // TimestampWritable.decimalToTimestamp
     case DecimalType() =>
       buildCast[Decimal](_, d => decimalToTimestamp(d))
@@ -480,7 +480,7 @@ abstract class CastBase extends UnaryExpression with TimeZoneAwareExpression wit
     case TimestampType =>
       // throw valid precision more than seconds, according to Hive.
       // Timestamp.nanos is in 0 to 999,999,999, no more than a second.
-      buildCast[Long](_, t => microsToEpochDays(t, zoneId))
+      buildCast[Long](_, t => microsToDays(t, zoneId))
   }
 
   // IntervalConverter
@@ -1034,8 +1034,7 @@ abstract class CastBase extends UnaryExpression with TimeZoneAwareExpression wit
         val tf = JavaCode.global(
           ctx.addReferenceObj("timestampFormatter", timestampFormatter),
           timestampFormatter.getClass)
-        (c, evPrim, evNull) => code"""$evPrim = UTF8String.fromString(
-          org.apache.spark.sql.catalyst.util.DateTimeUtils.timestampToString($tf, $c));"""
+        (c, evPrim, evNull) => code"""$evPrim = UTF8String.fromString($tf.format($c));"""
       case CalendarIntervalType =>
         (c, evPrim, _) => code"""$evPrim = UTF8String.fromString($c.toString());"""
       case ArrayType(et, _) =>
@@ -1120,7 +1119,7 @@ abstract class CastBase extends UnaryExpression with TimeZoneAwareExpression wit
         val zid = getZoneId()
         (c, evPrim, evNull) =>
           code"""$evPrim =
-            org.apache.spark.sql.catalyst.util.DateTimeUtils.microsToEpochDays($c, $zid);"""
+            org.apache.spark.sql.catalyst.util.DateTimeUtils.microsToDays($c, $zid);"""
       case _ =>
         (c, evPrim, evNull) => code"$evNull = true;"
     }
@@ -1247,7 +1246,7 @@ abstract class CastBase extends UnaryExpression with TimeZoneAwareExpression wit
         zoneIdClass)
       (c, evPrim, evNull) =>
         code"""$evPrim =
-          org.apache.spark.sql.catalyst.util.DateTimeUtils.epochDaysToMicros($c, $zid);"""
+          org.apache.spark.sql.catalyst.util.DateTimeUtils.daysToMicros($c, $zid);"""
     case DecimalType() =>
       (c, evPrim, evNull) => code"$evPrim = ${decimalToTimestampCode(c)};"
     case DoubleType =>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/datetimeExpressions.scala

@@ -895,7 +895,7 @@ abstract class ToTimestamp
     } else {
       left.dataType match {
         case DateType =>
-          epochDaysToMicros(t.asInstanceOf[Int], zoneId) / downScaleFactor
+          daysToMicros(t.asInstanceOf[Int], zoneId) / downScaleFactor
         case TimestampType =>
           t.asInstanceOf[Long] / downScaleFactor
         case StringType =>
@@ -975,7 +975,7 @@ abstract class ToTimestamp
           boolean ${ev.isNull} = ${eval1.isNull};
           $javaType ${ev.value} = ${CodeGenerator.defaultValue(dataType)};
           if (!${ev.isNull}) {
-            ${ev.value} = $dtu.epochDaysToMicros(${eval1.value}, $zid) / $downScaleFactor;
+            ${ev.value} = $dtu.daysToMicros(${eval1.value}, $zid) / $downScaleFactor;
           }""")
     }
   }
@@ -1242,10 +1242,10 @@ case class DateAddInterval(
     if (ansiEnabled || itvl.microseconds == 0) {
       DateTimeUtils.dateAddInterval(start.asInstanceOf[Int], itvl)
     } else {
-      val startTs = DateTimeUtils.epochDaysToMicros(start.asInstanceOf[Int], zoneId)
+      val startTs = DateTimeUtils.daysToMicros(start.asInstanceOf[Int], zoneId)
       val resultTs = DateTimeUtils.timestampAddInterval(
         startTs, itvl.months, itvl.days, itvl.microseconds, zoneId)
-      DateTimeUtils.microsToEpochDays(resultTs, zoneId)
+      DateTimeUtils.microsToDays(resultTs, zoneId)
     }
   }
 
@@ -1261,10 +1261,10 @@ case class DateAddInterval(
          |if ($i.microseconds == 0) {
          |  ${ev.value} = $dtu.dateAddInterval($sd, $i);
          |} else {
-         |  long $startTs = $dtu.epochDaysToMicros($sd, $zid);
+         |  long $startTs = $dtu.daysToMicros($sd, $zid);
          |  long $resultTs =
          |    $dtu.timestampAddInterval($startTs, $i.months, $i.days, $i.microseconds, $zid);
-         |  ${ev.value} = $dtu.microsToEpochDays($resultTs, $zid);
+         |  ${ev.value} = $dtu.microsToDays($resultTs, $zid);
          |}
          |""".stripMargin
     })