Let me start answering this question with a story:
This happened the last time I was traveling by train. The train started from Hyderabad at 10:30 PM, at the next stop 2 people(a guy & a girl) came and they both claimed that a certain seat(Let’s say seat no. 11) is theirs.
Guy: This is my seat, check my ticket. I have booked it from this stop to XYZ location.
Girl: No this is my seat, You can check my ticket also though my reservation is not from this stop but from the next stop, my destination is also XYZ location.
They both showed the ticket to some other passengers and they both had the confirmed printed ticket of the same train for the same day for the same seat.
Guy: I am gonna sit on this seat as I booked my tickets long back.
Girl: I am gonna sit on this seat as it doesn’t matter who booked it before, I booked it 2 weeks back & since I was not getting confirmed ticket from this stop so I booked from next stop but this seat is mine.
Meanwhile, other passengers started with their analysis that there must have been a mistake from the railways and they booked the same seat for 2 people etc.
I was thinking, I am traveling by train for so many years and never heard railway booking the same seat for 2 people and being a software engineer myself I thought IRCTC definitely won’t have this big bug otherwise it would cause problems to lots of people every day.
Out of curiosity, I asked if they could show the tickets to me as well. I saw the same thing, they both have the same seat number of the same train for the same day but one thing was different that’s the onboarding station and time.
When I saw the girl’s ticket carefully, she booked the ticket from the next stop(as she was already claiming) and the onboarding time for the next stop was 12:02 AM. So actually she booked the ticket of the previous day as the date changes at 12:00 AM.
It took almost 15 minutes for me to convince the girl that she booked the ticket for the wrong day.
So what everyone should remember is “Date changes at midnight 12:00 AM”. :p