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

### Instruction:
How many different original air dates does the episode with series number 13 have?

### Inputs:
table_name: table, columns: ['Series No.', 'Season No.', 'Title', 'Directed by', 'Written by', 'Original air date', 'U.K. viewers (million)']

### Response:
SELECT COUNT Original air date FROM table WHERE Series No. = 13