Modify a PowerShell script for a SharePoint site collection backup

I have written a PowerShell script for backup of a SiteCollection, I want to modify it so it saves backup files (.bak) for 7 days so the backup folder has backups for 7 days. The scripts will run every day (24h). It´s not much left to do so I offer 20 dollars for this task. The one that can deliver it fastest gets the project. I will provide the script file to the one that is awarded the project.