Question

Can someone explain what the second d$year is not 1?

> d = as.POSIXlt("1900-01-01")
> d$year
[1] 0
> d$mon = d$mon + 12
> d
[1] "1901-01-01"
> d$year
[1] 0
>

Contrast with this:

> d = as.POSIXlt("1900-01-01")
> d
[1] "1900-01-01"
> d$year
[1] 0
> d$year = d$year + 1
> d
[1] "1901-01-01"
> d$year
[1] 1
>

Answer 1

POSIXlt objects are lists. You changed the mon element of the list. That does not change the year element of the list.

d <- as.POSIXlt("1900-01-01")
unclass(d)
d$mon <- 12
unclass(d)

If you want your change to alter any/all of the other list elements, convert it to POSIXct then back to POSIXlt.

unclass(as.POSIXlt(as.POSIXct(d)))