As per my understanding a set directly hits the data source table. If your filter condition has different set of output when compared to over all output, tableau is going to showcase only the intersection of filter conditions output and over all output with out any filter condition.
In this case, make your date filter as context filter by clicking on "Add to Context" option. This will generate a temporary table with your new date filter condition and this temporary table acts as a data source table for your view and so you will see the correct output.
from your description i think you should make the date filter part of the context, do this by right clicking on the pill in the filter shelf and 'add to context' it should turn grey and your table calcs should now work.
if not this then i would ask you include a workbook with your next post as it would be a lot easier to fix something we can see and touch.
Thank you very much