How to calculate YTD savings from the given table

Hello,

I am trying to calculate the YTD variance for my sample table shown below. 

Table Name is "Project Costs" 

I want to calculate YTD variance for this table

The formula is YTD variance = (Current month Variance) - (December month's variance of last year)

I used the below DAX calculation 

YTD_Variance = SUM('Project Costs'[Variance]) - CALCULATE(SUM('Project Costs'[Variance]), PREVIOUSMONTH('Date'[Date]))

This calculation works for January 2018 month, but for February 2018 it is showing wrong calculation as shown below.

Please help me correct DAX calculation 

PS: I have the date dimension included. 

Thanks

Bhavesh