Hi Ronald,
You can use the formatDateTime function to get the month from a date.
Document link: https://docs.mendix.com/refguide/parse-and-format-date-function-calls/
formatDateTime($YourDateVariable, 'MM') returns the month number as a string, e.g., '10' for October.
parseInteger(formatDateTime($YourDateVariable, 'MM')) returns the month as an integer from 1 to 12.
Have a look at the Parse and Format Date function calls in the Mendix documentation here; e.g. parseInteger(formatDateTime($yourDateTime, 'M')); Mendix uses the SimpleDateFormat, so you can find Date and Time patterns here.
You must use : instead of / for divisions as / is reserved for object/member navigation. Or use the div operator instead such as $value1 div $value2 which equals $value1 : $value2