Find the next monthly expiration date
When working with DateTime it's usually a good idea to not use a hardcoded DateTime.Now. This makes testing such methods very difficult because you cannot mock it so you have to write very tricky tests all with Now. You allso cannot easily switch to UtcNow etc. and you have to know which now it uses internally.

A much better solution would be to use another parameter like DateTime referenceDate. This way you can use any combinations of the two values and create more predictable and easier to understand tests.

Additionally I'd make this method static because it does not access any class fields etc.



Oh, I also need to add that you use excelent variable names! This is exactly how it should be done! ;-)