A bit long winded,but gives the answer you need. There probably is a better way of doing it.
=ToDate("01/"+(ToNumber(FormatDate(RelativeDate((ToDate("01/"+(ToNumber(FormatDate(CurrentDate();"MM")))+"/"+ToNumber(FormatDate(CurrentDate();"yy"));"dd/MM/yyyy"));-1);"MM")))+"/"+ToNumber(FormatDate(RelativeDate((ToDate("01/"+(ToNumber(FormatDate(CurrentDate();"MM")))+"/"+ToNumber(FormatDate(CurrentDate();"yy"));"dd/MM/yyyy"));-1);"yy"));"dd/MM/yyyy")