The following will return the date of the last day of the month:
If month({DateField}) in [1,3,5,7,8,10,12] then date(year(DateField}), month({Datefield}),31) else
If month({DateField}) in [4,6,9,11] then date(year(DateField}), month({Datefield}),30) else
If remainder(year({DateField}),4)=0 then date(year(DateField}), month({Datefield}),29) else
//above handles leap years
date(year(DateField}), month({Datefield}),28) else
If this is not what you are looking for let me know.
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