Create hourly intervals without regard to day-month-year in R - r

Question

I have a list of dates as this:
"2014-01-20 18:47:09 GMT"
"2014-01-20 18:46:59 GMT"
"2014-01-20 18:46:59 GMT"
"2014-01-20 18:46:59 GMT"
"2014-01-20 18:46:41 GMT"
I used this code to split the dates in four-hour intervals
data.frame(table(cut(datenormord, breaks = "4 hour")))
Results are these:
2013-07-22 06:00:00 144
2013-07-22 11:00:00 268
2013-07-22 16:00:00 331
2013-07-22 21:00:00 332
What I want is to see how many observations there are in each interval of four hours but not taking account of days months and years. For example I would like to see how many observations there are from 00:00 to 04:00 by adding observations of everyday of every year contained in my dataset
For example i want something like this:
01:00:00 1230
06:00:00 2430
11:00:00 3230

Answer 1

You can try removing the dates from your date using strftime then reformatting them to a date, which will just add the current day, year and month to all the datapoints. You can them break and count like you posted.
datenormord<-c("2014-01-20 01:47:09 GMT", "2014-01-20 07:46:59 GMT","2014-01-20 13:46:59 GMT" ,"2014-01-20 18:46:59 GMT" ,"2014-01-20 18:46:41 GMT")
datenormord<-strftime(as.POSIXlt(datenormord), format="%H:%M:%S")
datenormord<-as.POSIXlt(datenormord, format="%H:%M:%S")
result<-data.frame(table(cut(datenormord, breaks = "4 hour")))
You can remove the date in the final data frame as well:
result$Var1<-with(result,format(strftime(Var1,format="%H:%M")))