Filter on Calculated Column Working Incorrectly.

My data contains a field [StartDate].

This is coming from a SQL Server table source and is a datetime type.

To show how old each record is, I created a view in Data Insight, and added a column thus :

    Column [Age] =  date_diff(  ###.StartTime, to_date( sysdate(), 'YYYY.MM.DD'), 'DD')

I also only want to see items <=30 days old, so I added a filter in a similar fashion :

     date_diff(  ###.StartTime,to_date( sysdate(),'YYYY.MM.DD'), 'DD') <= 30

Now the interesting thing is the resulting data preview.

I am staring at my calculated column [Age] , and seeing values of 31 staring right back at me !?

Here's the values (SystemTime & StartTime) from 1 row , put into Excel and the Age recalculated:

.. so given that, the data preview shows that the Age is being calculated correctly.

Yet the filter is not working correctly.

Any ideas !!??