read_var doesn't automatically convert dates to seconds since start date

The function `read_var` doesn't automatically convert dates to seconds since start date.

This isn't a problem right now, since `read_var` is only called directly from `SimDir` and the diagnostics that are saved use the convention that time is seconds since start date rather than the dates themselves.

Start by locating the read_var implementation and its call sites from SimDir, then inspect how date values and the seconds-since-start-date convention are handled. Done means read_var automatically converts dates to seconds since the start date without disrupting the diagnostics convention.

Written by the indexing model from the issue text.