There is no rule of 12pm for common users.
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Tatkal Ticket Booking Time Changed :: July 10 - 2012
Indian railways announced that will revise the Tatkal ticket booking time changed from 8 AM to 10 AM on the previous day of journey from train originating station. That is we can book the Tatkal ticket after 10 AM only from July 10 -...
more... 2012. Indian Railways also announced that will the agents can able to book the Tatkal ticket from 12 AM only, this includes Indian Railways Catering & Tourism Corporation (IRCTC) and Railway Travel Service Agents (RTSAs).