"Are you flexible about time?"
Though this is a almost word for word translation of the Viet sentence, the English is absurd. 'Linh hoạt về thời gian' should be in idioms, maybe?
Or perhaps "Are you flexible with your time?"
Yes I assume so. Perhaps in English it should be "is your time flexible?"
My wager on most relevant english phrase would be "do you have a flexible schedule", I don't think I've ever heard 'flexible time', though I may be wrong.