Two short labels, one important clock
Earnings calendars often use BMO for before market open and AMC for after market close. If a company publishes at 7:00am before a 9:30am opening, regular-session investors can react when the market opens that morning. If it publishes at 4:15pm after a 4:00pm close, the next day's regular session is the first one fully following the news.
The exact clock times vary by exchange, and labels can be incomplete. The useful principle is not the abbreviation itself. It is whether the information was public before or after the price used to measure a reaction.
Why the closing price can be wrong
Suppose results appear fifteen minutes after Monday's close. Measuring from Monday morning to Monday's close would capture a day when the results were still unknown. Calling that the earnings reaction would place the outcome before the event.
A defensible daily comparison starts from the last legitimate close before publication and ends at the close of the first regular session in which the news was public. For an AMC release, that is normally the next trading session. Weekends and market holidays move the date further.
What about trading outside regular hours?
Prices can move in pre-market or after-hours trading. These sessions may have fewer participants, lower volume and wider spreads. They can provide useful information, but an intraday study needs suitable timestamped data and an explicit rule for which price is valid.
Mixing an after-hours price for one event with a regular close for another can make comparisons inconsistent. The same event clock and price source should apply throughout a study.