Variable change with time

I have a question about variable change with time.
I try to make a mass transfer model. The inflow concentration change with time.

If t <= 1000s then C=1mol/m^3
else C=0

I upload the data and make a Interpolation Function. Function name: int1

Then I set the Inlet Concentration as int1. It shows int1 is not variable.

How can I make this kind of variable?

Thanks in advance

Dylan