Function stan_model refers to field Date of packageDescription which doesn't exist

Function stan_model in rstan.R refers to field Date in the return value of packageDescription, and there is no such field.

R function packageDescription just returns whatever is in the DESCRIPTION file in the installed package, so the available fields depend on what was written there. The R version I am using (R 4.0.3 on macOS) doesn't write a field named "Date", so there is no such field available in the value returned by packageDescription.

I'm unable to construct a small, reproducible example. I bumped into the error when I was working with an existing model, but the error goes away after removing the .rds file (the reference to packageDescription("rstan")$Date occurs when checking the modification time of the .rds file). I wasn't able to construct a smaller example which showed the error -- the line of code in question is nested within some conditionals and I wasn't able to to deduce what combination of circumstances would lead to packageDescription("rstan")$Date being evaluated.

Maybe packageDescription("rstan")$Date should be changed to asDateBuilt (packageDescription ("rstan")$Built) since I see that there is a field Built in DESCRIPTION. However, if the fields stored in DESCRIPTION can vary from version to version of R, then I don't know how to future-proof this test.

packageVersion("rstan") reports 2.26.1.

R.version.string reports "R version 4.0.3 (2020-10-10)".

Operating system: macOS Catalina 10.15.7.

Start in rstan.R at stan_model, where packageDescription("rstan")$Date is used while checking an .rds file's modification time. Inspect the surrounding conditionals and compare the available DESCRIPTION fields, including Built, on the reported R version. Done means the existing model-cache path no longer assumes a missing Date field and still performs its timestamp check correctly.