While I'm late to this question, this is possibly of use to someone. If you want to know how long ago the last restore point was, the restore point creation date property needs to be converted before use.
(Get-ComputerRestorePoint | select -first 1 @{label="Date"; Expression={$_.ConvertToDateTime($_.CreationTime)}}).Date
and to get how long ago it was you need a timespan calculation:
new-timespan -start ((Get-ComputerRestorePoint | sort CreationTime -descending | select -first 1 @{label="Date"; Expression={$_.ConvertToDateTime($_.CreationTime)}}).Date) -end (get-date)
hopefully this will save someone a bit a frustration.