If I have understood it correctly, you are trying to convert a String representing a given date, to another type.
Note: (As @Samson Scharfrichter has mentioned)
There are a few ways to do it. And you are close to the solution. I would use the CAST (which converts to a DATE_TYPE):
SELECT cast('2018-06-05' as date);
Result: 2018-06-05 DATE_TYPE
or (depending on your pattern)
select cast(to_date(from_unixtime(unix_timestamp('05-06-2018', 'dd-MM-yyyy'))) as date)
Result: 2018-06-05 DATE_TYPE
And if you decide to convert ISO8601 to a date type:
select cast(to_date(from_unixtime(unix_timestamp(regexp_replace('2018-06-05T08:02:59Z', 'T',' ')))) as date);
Result: 2018-06-05 DATE_TYPE
Hive has its own functions, I have written some examples for the sake of illustration of these date- and cast- functions:
Date and timestamp functions examples:
Convert String/Timestamp/Date to DATE
SELECT cast(date_format('2018-06-05 15:25:42.23','yyyy-MM-dd') as date); -- 2018-06-05 DATE_TYPE
SELECT cast(date_format(current_date(),'yyyy-MM-dd') as date); -- 2018-06-05 DATE_TYPE
SELECT cast(date_format(current_timestamp(),'yyyy-MM-dd') as date); -- 2018-06-05 DATE_TYPE
Convert String/Timestamp/Date to BIGINT_TYPE
SELECT to_unix_timestamp('2018/06/05 15:25:42.23','yyyy/MM/dd HH:mm:ss'); -- 1528205142 BIGINT_TYPE
SELECT to_unix_timestamp(current_date(),'yyyy/MM/dd HH:mm:ss'); -- 1528205000 BIGINT_TYPE
SELECT to_unix_timestamp(current_timestamp(),'yyyy/MM/dd HH:mm:ss'); -- 1528205142 BIGINT_TYPE
Convert String/Timestamp/Date to STRING
SELECT date_format('2018-06-05 15:25:42.23','yyyy-MM-dd'); -- 2018-06-05 STRING_TYPE
SELECT date_format(current_timestamp(),'yyyy-MM-dd'); -- 2018-06-05 STRING_TYPE
SELECT date_format(current_date(),'yyyy-MM-dd'); -- 2018-06-05 STRING_TYPE
Convert BIGINT unixtime to STRING
SELECT to_date(from_unixtime(unixtime,'yyyy/MM/dd HH:mm:ss')); -- 2018-06-05 STRING_TYPE
Convert String to BIGINT unixtime
SELECT unix_timestamp('2018-06-05 15:25:42.23','yyyy-MM-dd') as TIMESTAMP; -- 1528149600 BIGINT_TYPE
Convert String to TIMESTAMP
SELECT cast(unix_timestamp('2018-06-05 15:25:42.23','yyyy-MM-dd') as TIMESTAMP); -- 1528149600 TIMESTAMP_TYPE
Idempotent (String -> String)
SELECT from_unixtime(to_unix_timestamp('2018/06/05 15:25:42.23','yyyy/MM/dd HH:mm:ss')); -- 2018-06-05 15:25:42 STRING_TYPE
Idempotent (Date -> Date)
SELECT cast(current_date() as date); -- 2018-06-26 DATE_TYPE
Current date / timestamp
SELECT current_date(); -- 2018-06-26 DATE_TYPE
SELECT current_timestamp(); -- 2018-06-26 14:03:38.285 TIMESTAMP_TYPE