Flight Changi Airport departs from Singapore Changi Airport at 20:40 and arrives at San Francisco International Airport at 21:20.
The flighttime is about 15h 40m. The distance travelled ("as the bird flies"): 15083 Km. So the average speed is roughly 963 Km.
There is an 15 hour time difference between Singapore and San Francisco
The best day to arrive at Singapore is Friday and it would be best to avoid Thursday, as prices are on average 135.67% higher than on Friday. (Average prices, based on 125556 datapoints.)
We have a lot more statistics for flights UA 28 and between Singapore and San Francisco.