How to specify to R that I want to import certain columns with a particular class?

See the {readr} package—in conjunction with the {lubridate} package it should bring in your data in usable form if you first export it from Excel to a csv file.

The readr::read_csv() function does a reasonably good job at guessing the appropriate column type. I find that dates in Excel are pretty hopeless, and I expect them to be brought in as strings to be parsed with lubridate::mdy(VARNAME) for date strings such as 17/12/2021, for example.

If you need more specific help, see the FAQ: How to do a minimal reproducible example reprex for beginners.