The following methods fail in restricted execution
mode, because they try to import the time module:
    datetime.datetime.strftime
    datetime.datetime.strptime
    datetime.datetime.timetuple
    datetime.datetime.utctimetuple
    datetime.time.time
    datetime.date.timetuple

Example of the problem:

>>> import datetime
>>> script = 'print dt; print dt.strftime("x")'
>>> exec script in {'dt':datetime.datetime(2006,1,1)}
2006-01-01 00:00:00
x
>>> exec script in
{'dt':datetime.datetime(2006,1,1),'__builtins__':{}}
2006-01-01 00:00:00
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<string>", line 1, in ?
KeyError: '__import__'

The attached adjusts the datetime module so that the
time module is imported in the initialisation of the
datetime module.  This allows these methods to be used
in restricted execution mode.

If the time module is not available, then the datetime
module will not be importable either after this patch.
 Previously the datetime module would be importable but
the methods listed above would raise exceptions.  If
this situation is worth considering then I can adjust
the patch accordingly.

I don't think that this is a good solution; datetime
is not the only module that does an import in such a
way (even some parts of typeobject.c are doing imports).
Note that some of the functions in time module that
datetime is calling are actually implemented in Python
and are also imported.

Your patch also introduces a memory leak (the time module
is never decrefed) and is missing tests.

I would like to remove datetime module dependency on time module altogether.  For example getting timestamp as a float and later break it into sec/usec just to satisfy time module API looks rather inefficient.

See issue9079.