For the number n provided, it starts with the time 00:00:00 of the current day and continues for the past n days.
It includes today also.
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Sample SOQL:
SELECT Id
FROM Account
WHERE CreatedDate = LAST_N_DAYS:365
Example – http://www.infallibletechie.com/2016/09/how-to-get-records-which-are-modified.html