How to add explicit types so I can use OVERLAPS?

I want to use the OVERLAPS keyword to compare two time intervals, but
I'm having trouble.

In the code below, I want to test if the time interval from 9:15 AM
through 9:45 AM overlaps the time interval from 8 AM through 10 AM.

This approach fails:

    >>> d = {'9:15 AM':datetime(2008, 8, 1, 9, 15),
    ...      '9:45 AM':datetime(2008, 8, 1, 9, 45),
    ...      '8 AM':datetime(2008, 8, 1, 8),
    ...      '10 AM':datetime(2008, 8, 1, 10)}

    >>> cursor.execute('select (%(9:15 AM)s, %(9:45 AM)s) '
    ...                'overlaps (%(8 AM)s, %(10 AM)s);', d)

    ------------------------------------------------------------
    Traceback (most recent call last):
      File "<ipython console>", line 1, in <module>
    ProgrammingError: function pg_catalog.overlaps(unknown, unknown,
    unknown, unknown) is not unique
    LINE 1: ...ct ('2008-08-01T09:15:00', '2008-08-01T09:45:00') overlaps
    (...
                                                                 ^
    HINT:  Could not choose a best candidate function. You might need to add
    explicit type casts.

Meanwhile, this approach works fine:

    >>> cursor.execute('select (TIMESTAMP %(9:15 AM)s, TIMESTAMP %(9:45 AM)s) '
                       'overlaps (TIMESTAMP %(8 AM)s, TIMESTAMP %(10 AM)s);', d)

    >>> cursor.fetchall()
    [(True,)]

For a lot of boring reasons, I don't want to go into all my code and add
the TIMESTAMP cast.

Is there anything else possible?

I'm happy to submit a patch if that is what it would take to add this
feature, and the patch will be considered.

Matt