You should use the “Modify Date” Activity. This makes it extremely easy to adjust dates, find the first day of previous months etc.
Note: You can also add multiple “Modifications” in the same activity to reach your desired result (i.e. Modification 1 - first day of the month, Modification 2 - last month).
What’s more: There’s a “Test” button that let’s you directly see and validate if the output would be as intended.