Your booking date will be 19th in Tatkal or even for advance booking (91 days in your case). This is because your train departs between 000hr next day.. Here's an example of your train:
Today is 18th, and I tried booking for 20th. Please note that even 21st is waitlisted showing that when trains are leaving after midnight, then the boarding point booking in TATKAL, and otherwise also, will be 3 days before date of journey, and in case of general booking it will be 91 days...
Accomodation...
more... Availability
Train No Train Name Date Source Station Destination Station Class
2163
CHENNAI EXPRESS
20- Apr- 2010
PUNE
MS
SL
Status Report
Serial No Date Status
1 20- Apr- 2010 WL 53/WL 37
2 21- Apr- 2010 WL 57/WL 47
3 22- Apr- 2010 AVAILABLE- 0181
4 23- Apr- 2010 AVAILABLE- 0181
5 24- Apr- 2010 AVAILABLE- 0181
6 25- Apr- 2010 AVAILABLE- 0181
RAC - Reservation Against Cancellation
WL - Wait List
RLWL - Remote Location Wait List
PQWL - Pooled Quota Wait List
REGRET/WL-No More Booking Permitted
I hope this clarifies.
Good wishes.