Both R and Excel assume a date is associated with every time object. In Excel, when you show a date it is an integer number of days since 1899-12-31 (due to a mistake made early in programming it). Whenever you show a time, it it merely displaying the time portion (fraction of a day) of a date/time. The date part of that value may or may not be 1899-12-31.
With this in mind, you are tilting at windmills hoping to import a "pure time" because no such thing exists in either program. You can choose to render a `POSIXct` as showing only the time portion when you convert it to character if you so choose.