Are the seconds of a date object stored in the object?

General
25

Yeah, predicting modify-in-place is hard (it's pretty rare); that example may be flawed given current R internals.

A more consistent example is in data.table, which works really hard to make sure it modifies in-place as much as possible (so as to be fast). Note the difference between base R and data.table:

df <- data.frame(x = 1:3)
lobstr::obj_addr(df)
#> [1] "0x7f8652344bf0"
lobstr::obj_addr(df$x)
#> [1] "0x7f8653d834b8"

df$x[[3]] <- 4L
lobstr::obj_addr(df)
#> [1] "0x7f8653959880"
lobstr::obj_addr(df$x)
#> [1] "0x7f8652fae988"

library(data.table)

dt <- data.table(x = 1:3)
lobstr::obj_addr(dt)
#> [1] "0x7f8652596400"
lobstr::obj_addr(dt$x)
#> [1] "0x7f8652e380c8"

dt[3, x := 4L]
lobstr::obj_addr(dt)
#> [1] "0x7f8652596400"
lobstr::obj_addr(dt$x)
#> [1] "0x7f8652e380c8"
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