For restorehistory, BOL says [restore_date] is the Date and time of the completion of the restore operation. My test shows otherwise. it appears to be the start datetime of restore operation. Inspired by Steve's post, I came up with this script to show the actual completion datetime of database restore operation. The idea behind is SQL error log always contain the an entry of restore operation in the end of db restore. something like "Restore is complete on database 'my_database'. The database is now available."
List database restore COMPLETION datetime
-- list database restore COMPLETION datetime
DECLARE @dt_start datetime = GETDATE() - 7 -- 7 days ago for most recent possible restore operation
DECLARE @dt_end datetime = GETDATE()
DECLARE @dbrestored TABLE (logdate datetime, processinfo varchar(25), infotext varchar(500))
INSERT INTO @dbrestored
EXEC xp_readerrorlog 0, 1, N'Restore is complete', N'The database is now available.', @dt_start, @dt_end, 'DESC'
SELECT SUBSTRING(infotext,34, (CHARINDEX ('''', infotext, 34)-34)) AS database_name
, logdate
, infotext
FROM @dbrestored