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.

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