I'm currently writing a report to assess the last backup time of full backups.  What I want to do is to report any savesets/clients that have not had a successful backup in the last week/month.

I thought the the Time To Duration operator would allow me to assess the number of days since the last full backup which I could then use to filter the results - this is meant to calculate the time difference between the value of the field and the end of the window. 

I use the Queue Start field (renamed to Last Full Backup) for the Time Field; I have set the Duration Field to Duration (which I understand is just the name of the output field).  I have tried changing the cast of Last Full Backup from date to anything which is time related with no success.  Each time I try to save the report it gives the error:

Time To Duration: please specify a valid time or integer field

I've verified that the report returns a date/time that appears to be valid (for example 17/12/10 21:59) so I'm not too sure why this operator has a problem.