RNS-261: Resolves review comments

core/src/main/java/org/apache/calcite/sql/dialect/BigQuerySqlDialect.java

@@ -1495,13 +1495,9 @@ private void unparseExtractFunction(SqlWriter writer, SqlCall call, int leftPrec
           && ((SqlBasicCall) firstOperand).getOperator().kind == SqlKind.MINUS) {
         SqlNode leftOperand = ((SqlBasicCall) firstOperand).getOperands()[0];
         SqlNode rightOperand = ((SqlBasicCall) firstOperand).getOperands()[1];
-        final SqlWriter.Frame epochFrame = writer.startFunCall("UNIX_SECONDS");
-        unparseOperandAsTimestamp(writer, leftOperand, leftPrec, rightPrec);
-        writer.endFunCall(epochFrame);
+        unparseExtractEpochOperands(writer, leftOperand, leftPrec, rightPrec);
         writer.print(" - ");
-        writer.startFunCall("UNIX_SECONDS");
-        unparseOperandAsTimestamp(writer, rightOperand, leftPrec, rightPrec);
-        writer.endFunCall(epochFrame);
+        unparseExtractEpochOperands(writer, rightOperand, leftPrec, rightPrec);
       } else {
         final SqlWriter.Frame epochFrame = writer.startFunCall("UNIX_SECONDS");
         unparseOperandAsTimestamp(writer, firstOperand, leftPrec, rightPrec);
@@ -1515,6 +1511,13 @@ private void unparseExtractFunction(SqlWriter writer, SqlCall call, int leftPrec
     }
   }
 
+  private void unparseExtractEpochOperands(SqlWriter writer, SqlNode operand,
+                                           int leftPrec, int rightPrec) {
+    final SqlWriter.Frame epochFrame = writer.startFunCall("UNIX_SECONDS");
+    unparseOperandAsTimestamp(writer, operand, leftPrec, rightPrec);
+    writer.endFunCall(epochFrame);
+  }
+
   private boolean isDateTimeCast(SqlNode operand) {
     boolean isCastCall = ((SqlBasicCall) operand).getOperator() == CAST;
     boolean isDateTimeCast = isCastCall