I'm still getting my head around date formulas, I have one to tell me the start date of the previous month:
=IFERROR(DATE(YEAR([Date Column]@row), MONTH([Date Column]@row) - 1, 1), DATE(YEAR([Date Column]@row) + 1, 1, 1))
This worked fine until we changed to a new year, what have I done wrong with this formula?