Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Date | Round | Opponent | Venue | Result | Attendance |
| --- | --- | --- | --- | --- | --- |
| 28 January 1989 | R3 | Raith Rovers | A | 1–1 | 10500.0 |
| 1 February 1989 | R3 R | Raith Rovers | H | 3–0 | 40307.0 |
| 18 February 1989 | R4 | Stranraer | H | 8–0 | 41198.0 |
| 21 March 1989 | QF | Dundee United | H | 2–2 | 42177.0 |
| 27 March 1989 | QF R | Dundee United | A | 1–0 | 21872.0 |
| 15 April 1989 | SF | St. Johnstone | N | 0–0 | 47374.0 |
| 18 April 1989 | SF R | St. Johnstone | N | 4–0 | 44205.0 |
| 20 May 1989 | F | Celtic | N | 0–1 | 72069.0 |
Question:
what is the date when the round is sf?
Answer:
15 April 1989