# métodos al guardar
def get_prep_value(self, value):
    if value is not None:
        value = u.get_strdatetime(value) #<-- bug here
    return value

def get_prep_value(self, value):
    if value is not None:
        value = u.get_strdatetime(datetime.strptime(value, '<format>'))
        value = value.strftime('%Y%m%d%H%M%S')
    return value