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Why are open times one minute late?
https://github.com/quantopian/trading_calendars
Due to its historical use in the Zipline backtesting system, trading_calendars will only indicate a market is open upon the completion of the first minute bar in a day.
Zipline uses minute bars labeled with the end of the bar, e.g. 9:31AM for 9:30-9:31AM.
As an example, on a regular trading day for NYSE:
9:30:00 is treated as closed.
9:30:01 is treated as closed.
9:31:00 is the first time treated as open.
16:00:00 is treated as open
16:00:01 is treated as closed
This may change in the future.
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