How do I compare compare dates and values?

It sounds like what you want is something along the lines of PARALLELPERIOD

https://support.office.com/en-us/article/PARALLELPERIOD-Function-f38ab5a7-d88c-48a8-9eb2-dc67d5aa3f40?ui=en-US&rs=en-US&ad=US

There are also some techniques here that might help:

http://www.daxpatterns.com/time-patterns/

Not sure what you have tried thus far.

I suppose you want to compare januar 2014 to januar 2015. If so, you can use this DAX formula:

Prior Year Same Month Sales = CALCULATE(
        SUM([Sales Amount]);
        DATESMTD(SAMEPERIODLASTYEAR(Calendar[DateKey])))