Change daylight saving time to standard time in R

Question

I am trying to convert local time (US Mountain Time Zone) to UTC. R seems to do some strange things regarding the transition from MDT to MST. Should this transition not occur at 2 o'clock local time? R does this at 1:10 in the morning, in my case (see conclusion below). My time is defined as the POSIXct series.

> subdata$Date[11:15]
[1] "2008-11-02 00:55:00 MDT" "2008-11-02 01:00:00 MDT" "2008-11-02
01:05:00 MDT" "2008-11-02 01:10:00 MST" "2008-11-02 01:15:00 MST"

five_min_data$Date <- as.POSIXct(as.character  
(five_min_data$Date),tz="America/Denver")

Any help regarding what I might be doing wrong would be greatly appreciated.