First of all there is no time decay for Futures. There is time value only for Options. I think it is called Theta in the Options Greeks.
Theta is function of Underlying price, Strike Price, Interest Rate, Time to Expiry, Dividend and Volatility. Now if the underlying is not moving, the only parameter which would change is the "time to expiry". When we say underlying it is the SPOT and not the Futures.
There are tools to calculate Theta or time decay value. I had provided in my OAT tool also.