[R] ggplot : scale_x_datetime issues

[ravi]

Dear R users,
My immediate problem is that I do not seem to understand how to use the function scale_x_datetime function in ggplot. My deeper problem is that I don’t understand the use of the POSIXct class.
 
My data looks like like the following :
T          val
2009-11-04 23:59:57   972.357117     
2009-11-05 00:02:21   969.389221     
2009-11-05 00:04:45   868.817749     
2009-11-05 00:07:09   805.593079     
2009-11-05 00:09:33   928.946106     
2009-11-05 00:11:57   1004.880249   
2009-11-05 00:14:21   969.154846     
2009-11-05 00:16:45   989.090942     
2009-11-05 00:19:09   1009.907837   
2009-11-05 00:21:33   1058.458374   
2009-11-05 00:23:57   1179.298096   
………………
……………….
2009-11-06 23:57:23   650.345631
 
 
I attempted to code with the following :
 
df<-read.table(file=file1,header=TRUE,sep="\t",colClasses=c("character","numeric"))
# Could not directly read in the time with the POSIXct class instead of as character
t1<-as.POSIXct(df$T)
# interested in a specific time frame
# first attempted without the CET in the limits
lim2<- c("2009-11-05 12:00:00 CET","2009-11-06 13:00:00 CET")
library(ggplot)
ggplot(data=df,aes(x=T,y=val))+geom_line()+scale_x_datetime(limits=lim2,format="%H")
 
I get the following error message :
#####
Error in if (length(range) == 1 || diff(range) == 0) { : 
  missing value where TRUE/FALSE needed
In addition: Warning message:
In get("transform", env = trans, inherits = TRUE)(trans, ...) :
  NAs introduced by coercion
######
I next tried to get a subset of the dataframe with the following :
 
df2<-df [df$T>"2009-11-05 11:57:33 CET" & df$T<"2009-11-06 12:59:57 CET",]
> head(df2)
[1] Tid1 Rok 
<0 rows> (or 0-length row.names)
 
I don’t seem to understand how I should work with the POSIXct class. And, this is just the first step in my analysis (with many more steps pending)..
I will appreciate any and all the help that I can get.
Thanking you,
Ravi