def at(self, timestamp):
        """
        Force the create date of an object to be at a certain time; This
        method can be invoked only on a freshly created Versionable object.
        It must not have been cloned yet. Raises a SuspiciousOperation
        exception, otherwise.
        :param timestamp: a datetime.datetime instance
        """
        # Ensure, it's not a historic item
        if not self.is_current:
            raise SuspiciousOperation(
                "Cannot relocate this Versionable instance in time, since it "
                "is a historical item")
        # Ensure it's not a versioned item (that would lead to some ugly
        # situations...
        if not self.version_birth_date == self.version_start_date:
            raise SuspiciousOperation(
                "Cannot relocate this Versionable instance in time, since it "
                "is a versioned instance")
        # Ensure the argument is really a timestamp
        if not isinstance(timestamp, datetime.datetime):
            raise ValueError("This is not a datetime.datetime timestamp")
        self.version_birth_date = self.version_start_date = timestamp
        return self
Force the create date of an object to be at a certain time; This
        method can be invoked only on a freshly created Versionable object.
        It must not have been cloned yet. Raises a SuspiciousOperation
        exception, otherwise.
        :param timestamp: a datetime.datetime instance