Continuous variables in survival analysis

Hi,

I am currently working on a project that requires me to use a cox proportional hazard survival model- I have never used this before. What I am struggling with is how to make a continuous variable such as interest rates or real GDP an exposure variable when it changes over the time period of survival.

Context: I am trying to model a subdivision process with 4 stages, I want to know how economic indicators affect the time to 'death' (time to completion of a stage), this process can take 5-10 years so the interest rate the properties are exposed to changes over this time. Initially I thought about anualizing the data but that does not work when the development survives over several years. I am using excel and Stata.

Any ideas appreciated, I hope I have explained this well enough.

Thanks,