I want to say thanks to all of the Proxmox staff, and contributors. My containers no longer stay locked after backups. This has been a big pain for a while now, and I'm so glad it's fixed!
Good morning,
If you take a look at the below link it lays out the format for scheduling backups.
https://pbs.proxmox.com/docs/calendarevents.html
For your example though, if you wanted to perform a backup every 30 minutes you can use the following syntax: *:00/30