Task: Question Understanding
Input (English): Context : South African Airways Flight 228 was a scheduled flight of the Boeing 707-300C Pretoria, registration ZS-EUW, on 20 April 1968 from Johannesburg to Heathrow via Windhoek, Luanda, Las Palmas and Frankfurt. 
Question : What is the full date that South African Airways Flight 228 crashed on?
Output (English): point in time