Symptoms:

• The Disk Space|Snapshot|Number of Days Old property value shows as -1.

VMware vRealize Operations Manager 6.6.x
VMware vRealize Operations Manager 7.5.x
VMware vRealize Operations Manager 6.7.x
VMware vRealize Operations Manager 7.0.x

Any virtual machine with a snapshot will have the Disk Space|Snapshot|Number of Days Old property.
This property will grow +1 every day within a 24 hour period while a snapshot exists on the virtual machine.
Once you delete all snapshots of a virtual machine, its Disk Space|Snapshot|Number of Days Old property will be set to -1, and remain -1 until you create a new snapshot for that virtual machine.
After you create a new snapshot the value will start growing again, starting from 1 and adding +1 every 24 hour period while the snapshots exist.

This is expected behavior in vRealize Operations Manager.