[SPARK-16609] Add to_date/to_timestamp with format functions

## What changes were proposed in this pull request?

This pull request adds two new user facing functions:
- `to_date` which accepts an expression and a format and returns a date.
- `to_timestamp` which accepts an expression and a format and returns a timestamp.

For example, Given a date in format: `2016-21-05`. (YYYY-dd-MM)

### Date Function
*Previously*
```
to_date(unix_timestamp(lit("2016-21-05"), "yyyy-dd-MM").cast("timestamp"))
```
*Current*
```
to_date(lit("2016-21-05"), "yyyy-dd-MM")
```

### Timestamp Function
*Previously*
```
unix_timestamp(lit("2016-21-05"), "yyyy-dd-MM").cast("timestamp")
```
*Current*
```
to_timestamp(lit("2016-21-05"), "yyyy-dd-MM")
```
### Tasks

- [X] Add `to_date` to Scala Functions
- [x] Add `to_date` to Python Functions
- [x] Add `to_date` to SQL Functions
- [X] Add `to_timestamp` to Scala Functions
- [x] Add `to_timestamp` to Python Functions
- [x] Add `to_timestamp` to SQL Functions
- [x] Add function to R

## How was this patch tested?

- [x] Add Functions to `DateFunctionsSuite`
- Test new `ParseToTimestamp` Expression (*not necessary*)
- Test new `ParseToDate` Expression (*not necessary*)
- [x] Add test for R
- [x] Add test for Python in test.py

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: anabranch <wac.chambers@gmail.com>
Author: Bill Chambers <bill@databricks.com>
Author: anabranch <bill@databricks.com>

Closes #16138 from anabranch/SPARK-16609.

1 files changed, 9 insertions, 0 deletions

diff --git a/python/pyspark/sql/tests.py b/python/pyspark/sql/tests.py
index 86cad4b363..710585cbe2 100644
--- a/python/pyspark/sql/tests.py
+++ b/python/pyspark/sql/tests.py
@@ -1807,6 +1807,8 @@ class SQLTests(ReusedPySparkTestCase):
         self.assertTrue("+" in functions)
         self.assertTrue("like" in functions)
         self.assertTrue("month" in functions)
+        self.assertTrue("to_date" in functions)
+        self.assertTrue("to_timestamp" in functions)
         self.assertTrue("to_unix_timestamp" in functions)
         self.assertTrue("current_database" in functions)
         self.assertEquals(functions["+"], Function(
@@ -2189,6 +2191,13 @@ class HiveContextSQLTests(ReusedPySparkTestCase):
         # Regression test for SPARK-17514: limit(n).collect() should the perform same as take(n)
         assert_runs_only_one_job_stage_and_task("collect_limit", lambda: df.limit(1).collect())
 
+    def test_datetime_functions(self):
+        from pyspark.sql import functions
+        from datetime import date, datetime
+        df = self.spark.range(1).selectExpr("'2017-01-22' as dateCol")
+        parse_result = df.select(functions.to_date(functions.col("dateCol"))).first()
+        self.assertEquals(date(2017, 1, 22), parse_result['to_date(dateCol)'])
+
     @unittest.skipIf(sys.version_info < (3, 3), "Unittest < 3.3 doesn't support mocking")
     def test_unbounded_frames(self):
         from unittest.mock import patch