Hard to read your mind on what you don't understand but here goes; -

If you have a Total year measure then it must be based on either an ETL process populating an all year number, which does not seem likely, or a value that is aggregated and has its detail level set to the total level of your time hierarchical dimension.

Assuming it is the latter you need to copy it and have its copy set to the detail level of the time hierarchy.

Then you can create a new time series measure on this, that will use the AGO, YTD or whatever.

Then you will have the Year Total measure (as you do now), a detail equivalent that will only relate to the relevant time period, and a time series measure which will work, provided you have time in your analysis.

Finally you can then build a logical measure which makes the A - B functionality that you alluded to in your original question.

If this still does not make sense then please provide precise detail of what you are not understanding.

Note - I am saying to do all of the above on the RPD, business model layer (and presentation!)