Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
When was the 1991 world group I with the opponent of Li Fang?

### Inputs:
table_name: table, columns: ['Edition', 'Round', 'Date', 'Surface', 'Opponent', 'Result']

### Response:
SELECT Date FROM table WHERE Edition = 1991 world group i AND Opponent = li fang