There still doesn't seem to be a supported method of changing the new date picker wheels as of Xcode 12.1.
I've come up with this method to get around the issue:
func adjustDatePicker(wheel: XCUIElement, to newValue: String) -> Bool {		
		let x = wheel.frame.width / 2.0
		let y = wheel.frame.height / 2.0
		// each wheel notch is about 30px high, so tapping y - 30 rotates up. y + 30 rotates down.
		var offset: CGFloat = -30.0
		var reversed = false
		let previousValue = wheel.value as? String
		while wheel.value as? String != newValue {
				wheel.coordinate(withNormalizedOffset: .zero).withOffset(CGVector(dx: x, dy: y + offset)).tap()
				let briefWait = expectation(description: "Wait for wheel to rotate")
				briefWait.isInverted = true
				wait(for: [briefWait], timeout: 0.25)
				if previousValue == wheel.value as? String {
						if reversed {
								// we already tried reversing, can't find the desired value
								break
						}
						// we didn't move the wheel. try reversing direction
						offset = 30.0
						reversed = true
				}
		}
		
		return wheel.value as? String == newValue
}