intertemporal utility function usage : calculating consumption
This is the two-period budget constraint:


  C1 + C2/(1+r) = Y1 + Y2/(1+r)


Derivation is straightforward. On the LHS, you have the present value of consumption (considered during period 1), and on the RHS you have the present value of income. 

Intuitively, think about 1/(1+r) on the LHS as the price ratio between Good 1 and Good 2. Now you can solve the u-max problem as per usual.