Zope import: 'module' object has no attribute '_dt_reconstructor' - zope

Question

When trying to import some objects from my server (debian) on another machine (mac), I get this error:
Traceback (innermost last):
Module ZPublisher.Publish, line 126, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 46, in call_object
Module OFS.ObjectManager, line 620, in manage_importObject
Module OFS.ObjectManager, line 638, in _importObjectFromFile
Module ZODB.ExportImport, line 92, in importFile
Module transaction._transaction, line 260, in savepoint
Module transaction._transaction, line 257, in savepoint
Module transaction._transaction, line 690, in __init__
Module ZODB.Connection, line 1123, in savepoint
Module ZODB.Connection, line 587, in _commit
Module ZODB.ExportImport, line 176, in _importDuringCommit
AttributeError: 'module' object has no attribute '_dt_reconstructor'
The Zope and Python versions are exactly the same (Zope 2.13.19 and Python 2.7.3) on both machines. Re-importing on the same machine works fine.
I tracked down the object causing the problem, it is an instance of one of my custom classes. My problem is that I don't know where to look to fix the error.
One of the base classes of this class uses DateTime, but that does not cause any problems with the other classes that inherit from this base class.

Answer 1

Check your DateTime egg versions.
DateTime 2.12.7 added the function _dt_reconstructor, and you'll need to use at least that release in the 2.12.x series, or use version 3.0.1 or newer in the 3.0.x series to be able to unpickle DateTime instances again.
The 3.0.x series uses a much more efficient internal structure, leading to significant memory savings. However, 3.0.x DateTime code can read 2.x series pickles, but once you move to 3.x you cannot go back to 2.x as the format is not backwards compatible.