You have not mentioned which version of Monarch you are using but do you have the DateAdjust() function available?
would give you today's day, adding 0 years and 1 month.
Also FirstDay() might be useful?
lastday(today(),"month") will give you the last day of the current month.
will give you the first day of the next month.
Change today() to your date field and you should have all the options to play with. (Assuming you are not running an very old version of Monarch.)