As an example, let's say you're scheduling your campaign to be sent on a Wednesday at noon, and you're using a condition which contains the phrase "Did not perform event X in the past 2 days".
This period of time includes 2 calendar days before the day the campaign is scheduled to send, plus the amount of time between midnight and the time the campaign is sent on the day the campaign is sent.
Using the example campaign, this means that the time period covers Monday, Tuesday, and Wednesday from midnight to noon. So, you are targeting users who did not perform event X on Monday, Tuesday, or Wednesday before the campaign sent.
If you use a condition that includes the phrase "did not perform event X in the past 0 days", you will be targeting only the period of time between midnight on the day the campaign is scheduled to be sent, and the time that the campaign is sent. This will target users who did not perform event X on Wednesday before the campaign was sent.
If you use the condition "did not perform event X exactly 2 days ago", this refers to 2 calendar days ago. Using the same example campaign from above, this refers to Monday. This targets users who did not perform event X on Monday.