run your income statement for 04/01/19-04/30/19 and then in the compare another period, a custom option appears.  Set any date there.  Then change your income statement dates to 04/01/21-04/30/21 and the custom option for comparison stays.  Change that to 04/01/2019-04/30/2019 and you have what you need.

For some reason if you run it for current month, you don't get the option but this way tricks it to allow it.

I hope that helps!!