'Parameters
user_id: user id in the model
int or string
item_id: id of the item in the model
string or int
Returns
Retrieves the time at which a preference value from a user and item
was set, if known. Time is expressed in the usual way, as a number
of milliseconds since the epoch.'

def preference_time(self, user_id, item_id):

    raise NotImplementedError('cannot    instantiate    Abstract    Base    Class')
