Try like this
Date(Date#(MonthField, 'MMM'), 'MM') as MonthField
Match(MonthField, 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec') as MonthField
by using Mapping table.
Hope this helps you.
I just had the same problem. num(MonthField,'00') solves the problem within QlikView/Qlik Sense but the problem is that when I export to excel, the entry in the cell is '0' even though it displays '00'. I added the Text() function and that worked. Now it is ok in Qlik Sense and also after excel export. Final syntax is text(num(MonthField,'00'))