I use this method can get the first day of this month and the last day,
Reference DateUtils

DateTImePicker1. Date:=StartofTheMonth (Now);
DateTimePicker2. Date:=EndOfTheMonth (Now);

Format: 2020-10-01 2020-10-31

With what method can get the first day of next month and the last day

The result is: 2020-11-01 2020-11-30
November 31,