EOMONTH returns the last day of a month.
Using -1 gives the last day of the month before, and adding 1 returns the first day of the month

So, EOMONTH(today(),-1) returns Sep 30th, and adding 1 gives Oct 1st

The result can be obtained other ways ( like DATE) but it would have made the formula longer, or using >eomonth(ref,-1) instead of >=.

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