Supports date_add and date_sub function

[killme2008]

What problem does the new feature solve?

DATE_ADD(date, INTERVAL value interval_type)

and

DATE_SUB(date, INTERVAL value interval_type)

For example:

SELECT DATE_ADD('2015-06-15', INTERVAL '5 YEAR') AS Result; 

SELECT DATE_SUB(now(), INTERVAL '5 YEAR') AS Result; 

What does the feature do?

As a TSDB, we must have more functions to support date or time types.

Implementation challenges

No response