U can book Tatkal @ 00:30 for previous date opened Tatkal Quota ot for the coming morning quota!!
eg.
Travelling date: 10 july
Tatkal opening date: 8 July
On 8 July @ 00:30 (or before...
more... 8:00 am) u cant book tatkal fo 10 journey!!
but on 9 July @ 00:30, u can book tatkal for 10 (if berth remain available)