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You can use the built-in functions setDate, setHour, setMinute, and setSecond to get the first day of this month.
For the last day of this month, it is a similar formula with two more built-in functions addMonths and addDays. This will automatically cater to months ending on different days (28/29/30/31) — which I have used here 📅 How to schedule scenario for the last day next month (recurring)
Output
For more information, see https://www.make.com/en/help/functions/date---time-functions