Okay... Let me shed some light on this whole flight tracking thing...
Right now, Flight tracking is done by parsing your email on the device for confirmation mails from carriers and 3rd party travel sites. Unfortunately, these are all formatted differently and the information is not always where you think it is or in the format we can parse easy and all that mess, so this is something that is hit and miss and just over a mess, as stated in this thread already. We are working on that, and working very hard on it.
Now, you can tell Cortana to track a flight, but, for now, only on the date you ask. This is also being worked on.
You can tell Cortana to "Track Delta Flight 5535" and it will. But, for now, you cannot tell her to "Track Delta Flight 5535 on July 30th". This is being worked on and should be coming sometime in an update. No, I have no ETA.
I hope this helps clear things up a little. I know, cleared it up as much as Tar, but it's all I have right now.