I agree Omar's suggestion should work. Qlik Sense does some weird things with dates though since its stored as a dual (number and string). If it does not correctly identify your link_YearMonth as a date field it may still not work as expected.
The "easiest" way is to set a flag in the load script for LastMonth. I've created a field in load script that is MonthsFromCurrent to allow selection of last month or 2 months ago or 12 months, etc.that is in our standard calendar to allow for selection of any prior month (or period).