Combine one set of time series data with another when some dates are missing in one data set

Solution 1:

1. Create a new LibreOffice Base database (embedded hsqldb);
2. Create two tables (A, B) that have the same structure as your data columns;
3. In your spreadsheet, select and copy data columns Date-A and A;
4. Paste those two columns in Base into Table A (in table view, select table A and hit CTRL+V)
5. Repeat steps 3 and 4 for columns of B data (Date and Data);
6. Create a new Query in SQL mode:

SELECT "Date-A" AS "Date", "A", "B" FROM "A" JOIN "B" ON "A"."Date-A" = "B"."Date-B";

Result:

Additional Remarks: Don't try to create a new db based on your spreadsheet directly. Those databases don't support JOINs (AFAIK), you will receive strange error messages if you try... (as i did first)