The purpose behind this option is to allow you to tell the system the backup will run every x number of days. This will allow you to control a schedule for backups that take 2 days to run by example or they run every 14 days, etc.
1 - On the menu select Manage -> Schedules and then select Create Schedule.
2 - Give it a name and description for the schedule. Then check the box next to x number of days and put in the days.
If you're updating a single backup go to the edit screen and remove the schedule by clicking the x. If adding or changing the schedule by bulk edit it will automatically overwrite the schedule with the new parameters.
Select the new schedule and click Add new Schedule as shown below.
Then put in the start date of when you want the system to start for the backup. This can be anytime in the past or future for the start date. The system will then show the backup from that date moving forward according to x number of days that were set.
Once you select the first date, then click Save. Last date is only used when a backup is retired.
So for example the system would expect a backup on Friday May 5th and every 2 days from that point on by the example used.