Lubridate official cheatsheet wrote “imaginary”(in attach a screenshot), i think someone decide to call the function with that name because if substract one year to 29-02-2020 in can find a date, because 2019 is not a leap year.
Lubridate Developer write to me on github repository issue:
Yes. Unfortunately the original design of period arithmetics is not that most people expect. So your only option for now is to use %m+% or add_with_rollback functions. I will likely be changing the internals of lubridate by the end of the year and there might be a global user option to configure this behavior.