How to use ToDate() to concatenate month and year cells

Hi,

I have a universe giving me the year and month in the 'Number' format. I've attached a snapshot of the data below:

The measure values in the report are aggregated based on these dimensions. I want to concatenate these fields to produce something like:

Jan-15 and so on. I have tried using the ToDate and format function to accomplish this, but I end up getting a #ERROR everytime. Here is the expression:

=ToDate("" + [Year] + "-" + [Month] + "-01"; "yyyy-MM-dd")

Any suggestions on how to accomplish this?

Thanks!