Which dataset are you using as your primary source? This might happen if you're using the historical dataset as primary, as it will create a "pseudo left join" relationship on those dates. If this is the case, just flip them around so that your data set with historical + future dates is the primary data source.
If that doesn't work, it might be easier to diagnose the problem with a sample workbook.
I agree with Derek, it's much easier to understand the issue if we have a sample workbook to look at.
My hunch, without looking at the workbook is that your primary data source has a subset of the date range of your secondary data source. If you rebuild the view with the secondary as the primary and the primary as the secondary (confused yet), then the issue should be resolved.
Let us know!