Whenever you refresh seat availability, today's Train Availability status may come in as "Charted". That means the chart has already been prepared, as seen right now. When exactly was it prepared? Current time is 21:39. So we say it could be anytime "before 21:39". It may have been prepared at 21:00, 20:00, we don't know.
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Tomorrow if someone refreshes the availability at 21:15, and it once again says "Charted", then this will change to "before 21:15".
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Again later, if someone refreshes the availability at 22:00, and it says "Charted", then there won't be any change. since 21:15 < 22:00. The Charted time will remain at "before 21:15".