You have to format to date if it is not in correct format
Assuming my date field is in the format MM-DD-YYYY then
Check with this
Load Day(FieldName) AS day,Month(FieldName) AS Month;
Load Date#(DateField,'MM-DD-YYYY') AS FieldName From ExcelDatasource;
Load Day(Date#(DateField,'MM-DD-YYYY')) AS day,Month(Date#(DateField,'MM-DD-YYYY')) AS Month
To be able to use Date functions you should store your date in field in a right format.
Actually dates has two parts: text and numeric. It seems that you have just string in FIELD so date functions doesn't work.
So you should convert it to make a date from string. There are functions with # in the end that converts string into dual.
Above you can see the example of conversion in script.
You can also try it in a chart/listbox to make sure that the statement is correct (if you have loaded FIELD separately).