Maybe you are looking for function Monthname() ?
Returns a display value showing the month (formatted according to the MonthNames script variable) and year with an underlying numeric value corresponding to a timestamp with the first millisecond of the first date of the month. Shift is an integer, where the value 0 indicates the month which contains date. Negative values in shift indicate preceding months and positive values indicate succeeding months.
monthname ( '2001-10-19' ) returns 'Oct 2001' with an underlying numeric value corresponding to '2001-10-01 00:00:00.000'
monthname ( '2001-10-19', -1 ) returns 'Sep 2001' with an underlying numeric value corresponding to '2001-09-01 00:00:00.000'