fix year<-, and month<- about lubridate HOT 1 CLOSED

these functions call:
ISOdatetime(value, month(x), mday(x), hour(x), minute(x), second(x), tz(x))
when tz() returns NULL ISOdatetime fails

I rewrote tz() to have a supply option, tz(x, supply = F). If supply = T, tz will supply the time zone of the system time if the date does not have its own time zone attribute.

I then have month <- and year <- call
ISOdatetime(value, month(x), mday(x), hour(x), minute(x), second(x), tz(x, supply = T))
which works.

note: this method automatically assigns the system time zone to dates that do not have a time zone. I feel that this is acceptable because base R automatically does the same thing whenever someone creates a POSIXct date, or uses as.POSIXct on a POSIXlt date that doesn't have a time zone. (i.e. we'd have to go out of our way and over write some base R to keep this from happening).

from lubridate.