click here doubt clarified on wikipedia
Tatkal Booking starts one day in advance (reduced from 2 days) excluding the day of journey e.g. for a journey on 3rd, bookings would open at 10 am on 2nd.[1] However, the day of journey is defined as the day of chart preparation. So if the train starts e.g. on 3rd, and reaches the desired boarding station on 4th, the Tatkal booking will start on 2nd and not 3rd