date_convert allows illegal 0 month/day values for partial datetime

to reproduce:

1) create a view with a date filter with "year" granularity, and "text field" as the field type.
2) enter an absolute value for the date filter, ie "2015"
3) press the "update" button.

expected behavior: filter should update normally.

actual behavior: receive an error message for the entry in the field. it is not saved.

Attached a patch. The year is run through date_convert, which ensures that we aren't missing month/day/hour/minute/second values... by filling them with 0's. This is fine for most date formats, but not for datetime. The attached patch changes the behavior to set 01s instead if you're on datetime.