Currently, there is an inconsistency in Looker Studio that sometimes the count distinct does not work correctly when tied to a date filter. This is a flaw recognized and reported by the community, as can be seen in the following links: Link1; Link 2; Link 3.

A commonly used method by users to fix this problem is to perform a blend of the table being used with itself. If you are experiencing this issue, just follow the step-by-step guide below:

1 - In the Looker Studio taskbar, click on “Resource” and then on “Manage blends”;

2 - Next, click the “Add a Blend” button;

3 - In “Table 1,” search for the table you want to use; in this example, we will use the table “sales_city_state”;

4 - Now click the “Join another table” button and select the same table;

5 - In both tables, under “Dimensions,” first select the key to be used between the tables; in this case, the key is “orderid”;

6 - Now click on “Configure Join”;

7 - In “Join Operator,” select the “Left outer” option, and under “Join Conditions,” select the dimension key for both tables and click “Save”;

8 - Now, only in “Table 1,” click on “Date Range” and select the desired date option; in this case, we will use “creationdate”;

9 - Next, in “Table 2,” add in “Dimensions” the fields you want to use in the dashboard. Leave the period field blank!

10 - In “Data Source Name,” give a name that identifies the blend, then click “Save”.

11 - Done! Now when creating a new chart on your dashboard, just select the created blend, and the “count distinct” will work correctly with a date filter.