The stored procedure in question removes backup history entries from the msdb system database. You specify a datetime parameter that indicates the oldest backup history entry to keep.
I have two questions
1) Is the oldest date to keep history inclusive or exclusive? In other words, if I specify 1/1/2016 12:00PM as my date time, do backups made exactly at 1/1/2016 at 12:00PM get removed or not.
2) A backup history record in msdb has two dates...the datetime the backup started, and the datetime the backup completed. Which field does sp_delete_backuphistory check against?