How can I get value from a period of time

[userchatel]

Hi guys, how are you doing ? 

I am currently having a problem with a value I'm trying to show.  I want to make a comparison between the revenue figures of the period of time selected in a filter pane and the same period beforehand (if I select 2015, it shows me the revenues of 2015 in a KPI indicator and have a second KPIindicator show the revenues of 2014). 

I have a function that works quite well that can show me the value 2014 when I select 2015 in the filter pane and I also made it do it with months and weeks.

Now I want to  get the data of 2014 of the revenues, how should I do it. 

My function to get 2014 : 

=If(GetSelectedCount('Année')>1, Num(SubField(GetCurrentSelections('Année'), ' ',-1),'0.0','')-1,  		
	If(GetSelectedCount('Mois')>1,      	     			
		Num(SubField(GetCurrentSelections('Mois'), ' ',-1),'0.0','')-1,           	
	Num(SubField(GetCurrentSelections('SemaineVolet'), ' ',-1),'0.0','')-1          	
		)      
	)

My variable to show my revenue is VAL_CA_TOTAL.

Thank you guys in advance