basic difference between Date, Date(ISO format), Date(Unix timestamp) to create a field
Below explains three field types in details

Date


Called a 'datetime' field type in techno database speak. This format type is recomended for most installs because it takes advantage of the database's date handling functions without any conversion, which means faster queries.
Stores the date in the database's native date format (YYYY-MM-DD HH:MM:SS).
The data is in a human-readable form.
You can use it for incomplete dates, i.e. only a year or only a year and a month, and pad the other values with zeros, so it does not appear to be any more precise than it really is.


Date (ISO Format)


Stores the date in an ISO format (YYYY-MM-DDTHH:MM:SS). 
The data is in a human-readable form. 
List You can use it for incomplete dates, i.e.
only a year or only a year and a month, and pad the other values with
zeros, so it does not appear to be any more precise than it really
is.
It is a format that is internationally-recognized, and it is used
as-is on many web sites and in many applications.


Date (Unix Timestamp)


Stores the date as an integer.
Takes up less room in the database because it's smaller.
Often easier to use for date calculations because you can increase or decrease it just by adding or subtracting seconds.
It is the format used by php date functions.
It must be filled with a complete date -- year, month, day, hour, minute, second, so you sometimes have to arbitrarily set some of these values even if they are not applicable.


Taken Reference from Here