You can use =today()-30 to set a cell's value to the last calendar month. To create a column with a rolling 12 month you can iterate the cells in a column by 30.

  • =today()-30
  • =today()-60
  • =today()-90
  • =today()-120

31 day months and February will create some hiccups, but unless you need this column for every day use it will be fine.