[R] as.Date question

[MAL]

All!

This piece of code:

zzz1 <- as.POSIXct("1999-03-18", tz="CET")
zzz2 <- as.POSIXlt("1999-03-18", tz="CET")
zzz1 == zzz2
as.Date(zzz1)
as.Date(zzz2)

yields TRUE for "zzz1==zzz2", but the two dates returned by as.Date are 
different:

> as.Date(zzz1)
[1] "1999-03-17"
> as.Date(zzz2)
[1] "1999-03-18"

I'm using R 2.10.0.

Would be glad for any clarifications. Thanks!