[chaley]

The following program demonstrates that mi.get() returns a datetime object for a datetime custom column.

from calibre.ebooks.metadata.book.base import Metadata
from calibre.library.database2 import LibraryDatabase2
from calibre.utils.config import prefs

src = prefs['library_path']
db = LibraryDatabase2(src)

mi = db.get_metadata(1353, index_is_id=True)
dt = mi.get('#mydate')
print dt
print type(dt)
print dt.timetuple()

Output:

2011-11-23 09:01:10+01:00
<type 'datetime.datetime'>
time.struct_time(tm_year=2011, tm_mon=11, tm_mday=23, tm_hour=9, tm_min=1, tm_sec=10, tm_wday=2, tm_yday=327, tm_isdst=0)

Adding to kovid's suggestion, the delegate used for datetime custom columns is CcDateDelegate at line 246 of delegates.py. This delegate is used to both display and edit the value. The value itself comes from models.py, function data() (line 702) that calls datetime_type() (line 580).