'Retrieve the latest opt-in preference for the user,
across all orgs and preference keys.
Arguments:
user (User): The user whos preference was set.
Returns:
ISO-formatted datetime string or empty string'

def _latest_pref_set_datetime(self, user):

    pref = UserOrgTag.objects.filter(user=user).order_by('-modified')
    return (pref[0].modified.isoformat('    ') if (len(pref) > 0) else self.DEFAULT_DATETIME_STR)
