Dividing time series by datetime period? - r

Question

I have some problems splitting a datetime variable into two variables. My time series is a /hour/day/month count of a full year (about 360 days).
I would like to generate one variable that ranges from the 1st of each month to the 19th of each month and the second variable captures 20 to the rest of the month:
Format:
datetime hours var1 var2 var3
2011-01-1 00:00:00
2011-01-1 01:00:00
... ...
2011-01-1 23:00:00
2011-01-2 00:00:00
2011-01-2 01:00:00
... ...
2011-01-2 23:00:00
... ...
... ...
2011-01-20 01:00:00
...
2011-01-31 00:00:00
... ...
2011-12-30 00:00:00
2011-12-30 01:00:00
.. ..
Desired Format:
datetime1 datetime2 var1 var2 var2
2011-01-1 00:00:00 2011-01-20 00:00:00
2011-01-1 01:00:00 2011-01-21 01:00:00
.. .. .. ..
2011-01-19 00:00:00 2011-01-30 00:00:00
2011-01-19 01:00:00 2011-01-30 01:00:00
.. .. .. ...
.. .. .. ...
2011-12-19 00:00:00 2011-12-30 00:00:00
2011-12-19 01:00:00 2011-12-30 01:00:00
Originally, I was able to produce the datetime variable by:
rbind or rbind.fill ( plyr) two data frames with datetime1 and datetime2
df3<-rbind(df1,df2)
That is, the original version (two data frames) had these two variables but Im not able to seperate them now.
I just couldt formulate the code...

Answer 1

Try this to extract the day of the date:
xx <- as.Date("2014-12-31")
as.POSIXlt(xx)$mday
You can then use the date as a condition to attribute NA to one column and value to the other.
EDIT: Here's the more in-depth version.
#Setting up a replicable example
mydata <- as.data.frame(matrix(rnorm(90), ncol=3))
names(mydata)[1:2] <- paste0("time",1:2)
mydata$time1 <- as.Date(NA)
mydata$time2 <- as.Date(NA)
str(mydata)
#Getting 30 consecutive days:
datestring <- rep(Sys.time(), 30)
for(i in 1:30)
datestring[i]<- Sys.time() + 60*60*24*i
mydata <- cbind(datestring, mydata)
#Doing what I think you're trying to do:
for (i in 1:dim(mydata)[1]){
if(as.POSIXlt(mydata$datestring[i])$mday <=19)
{mydata$time1[i] <- mydata$datestring[i]}
else {mydata$time2[i] <- mydata$datestring[i]}
}