Hi,
is the url hard coded into the flash in the current version ? If that is the case and you can't edit the flash it will be hard to do anything. I some months ago had a flash that linked to a certain url. That url was viewable as cleartext in the binary code of the flash file. I changed it to another url with a binary editor. You could change it to a custom php script on your server that will check the time and redirect to whatever page you want.
It can be very difficult to edit the flash file with php but you could edit it once manually like mentioned above to use a custom script.
The only thing is that the new url string in the flash should have exactly the same length as the original one. That shouldn't be too hard by using some dummy data. You might run in trouble if the new url has to be longer as the original one.
I hope this doesn't sound too confusing 🙂