Update Database Fields Based On Date
As Martin Smith said in the comments, there is no Date/Time based trigger, therefore you have two options.


Batch Update - Use a batch process to update the data.  This requires determining how often the data needs to change and then creating a batch process that runs at least that often to update the data.  If the information changes once a day this might be acceptable, but as the update need gets more frequent this solution becomes less usable.
Dynamic Evaluation - As Martin Smith suggested, determine availability when the information is requested.  This can be in the select statement itself or you can use a computed column on the table.  In either case the data will be as up to date as it can be.