Consider passing through date-time objects in parser functions

[vspinu]

Beginner R users often pass date-time objects to parsing functions like ymd_hms. This is very inefficient and can cause unexpected behavior (for example ymd_hm would not parse a full date). Directly returning an appropriate date-time when passed in a date-time object would avoid such issues.

[hadley]

Probably worth also warning, to help people learn good practice.