Предполагая, что имя вашего параметра равно {? Date}
{@ начальная_дата} формула:
//extract MM/YYYY from 'MM/YYYY-MM/YYYY'
local stringvar MMYYYY := Split({?dates},"-")[1];
//extract MM from 'MM/YYYY'
local numbervar MM := ToNumber(Split(MMYYYY,"/")[1]);
//extract YYYY from 'MM/YYYY'
local numbervar YYYY := ToNumber(Split(MMYYYY,"/")[2]);
//return first day of selected date
Date(YYYY,MM,1);
{@ окончание_даты} формула:
//extract MM/YYYY from 'MM/YYYY-MM/YYYY'
local stringvar MMYYYY := Split({?dates},"-")[2];
//extract MM from 'MM/YYYY'
local numbervar MM := ToNumber(Split(MMYYYY,"/")[1]);
//extract YYYY from 'MM/YYYY'
local numbervar YYYY := ToNumber(Split(MMYYYY,"/")[2]);
//calculate first day of selected month, add a month, subtract a day, return last day of selected month
DateAdd("m",1,Date(YYYY,MM,1))-1;