Mr Sai Jinendra is absolutely right. Take the case of train no 12312/kalka mail. Departing time 23.55 from kalka. Today(27.11.11) opened tatkal for 28.11.11.
If a passenger want a tatkal ticket from Ambala to Kanpur for 29.11
2011 (Train departs from ambala at 02.20 next day i.e. 29.11.11) he can book tatkal ticket today. I have checked it just now , you can also check.