Numbers doesn't have Pivot Tables (a feature of Excel). Numbers does have Categories.
Assuming you mean Categories, then the Subtotals are per category.
It sounds as if you are also trying to filter by date. If so, as you have discovered, the subtotals don't reflect the filter.
If you want to group dates by month then you can do two "levels" of Category to get something like this:

You could also turn off Categories, first apply your filter, select the visible cells, command-c to copy, click in the upper left cell of an existing destination table, and command-v to paste. Then apply the Category and choose Subtotal in that new table. (This is much quicker than it sounds; very fast and efficient.)
Or you can calculate subtotals using the SUMIFS function.
SG