Hi @wellisi ,
You could use the following formula to get the last day of the month of Date of the entry:
addDays(concat(add(int(formatDateTime(body('Get_item')?['Datum'], 'yyyy')),if(equals(formatDateTime(body('Get_item')?['Datum'], 'MM'), '12'), 1, 0) ),'-',add(int(formatDateTime(body('Get_item')?['Datum'], 'MM')),if(equals(formatDateTime(body('Get_item')?['Datum'], 'MM'), '12'), -11, 1) ),'-01T00:00:00Z'),-1)
Note: You may need to replace body('Get_item')?['Datum'] when you test it, replacing it with the Dynamic content associated with "Date of the entry".
Then use addDays() function to add one to the date of the last day to get the date of the first day of next month:
addDays(outputs('Last_Day_of_the_Month'),1,'yyyy-MM-dd')
Image reference:
Please take a try and feel free let me know if you have any question.
Best Regards,
