Stunningly useless operation today, apparently dependent upon whether you have Awards in your account or not.
Logged in to my account that has two transition Standard awards recently created from the AirTran two-step, and looked for a one-way flight in March 2014, well before the award expiry, and got this:
Tried selecting many dates only to find no availability on any flight on that date. Eventually tried logging out and logging in using my wife's account that has no Awards of any form, and got this:
The dates I couldn't book from my account match with the 'unavailable' as seen from my wife's account.
A secondary data point is that the calendar, when viewed from the account with the Awards available, has no selector for Freedom vs. Standard available; when using the account with no Awards, that selector appears but doesn't work. It seems to be defaulting to the Freedom award chart and not giving the option to show Standard availability when viewed from an account with Awards to use. Using Firefox 25.0 on a Win Xp machine.
Seems the current mode of operation is now search for availability with an account without Awards, then hope that persists while you log out and back in to the account with Awards.