You can also use a pure numeric month field created in your script ( num(month(DATEFIELD)) as numMonth ), and filter your data using this field (don't forget to clear any calendar field that may conflict).
But even when using such a field, there is one potential problem at year bounderies. So it might be even better to use a date field instead of the month (maybe created by monthname() or monthstart()) and addmonths() function to step one month forward or back.
