Sequence of time(hour)

I tried:

seq(
     from=as.POSIXct("2012-1-1 0", tz="UTC"),
     to=as.POSIXct("2012-1-3 23", tz="UTC"),
     by="hour"
   )  

But I only get 1 hour(0:00:00) of the last day instead of 24 hours, actually any hour of the day resulted in only one hour(0:00:00), and I do want to have 2012-1-4.