Date conversion as.yearmon

Question

I have a dataframe that looks like this:

library(zoo)
head(monthly.station6)

                    [,1]
1995-02-28 00:00:00 2.07
1995-03-01 00:00:00 5.70
1995-04-30 01:00:00 0.65
1995-05-31 01:00:00 1.03
1995-06-30 01:00:00 0.77
1995-07-31 01:00:00 0.39

I am applying this code: monthly.station6[,0] <- as.yearmon(monthly.station6[,0]) to try to convert this into a year month format, but I think the fact that the date column is [,0] is preventing it? Not sure where I am going wrong, any help would be appreciated!

head(monthly.station6)

             [,1]
Feb 1995 2.07
Mar 1995 5.70
Apr 1995 0.65
May 1995 1.03
Jun 1995 0.77
Jul 1995 0.39

as requested dput(head(monthly.station6)):

structure(c(2.07, 5.7, 0.65, 1.03, 0.77, 0.39), .indexCLASS = c("POSIXct", 
"POSIXt"), .indexTZ = "", tclass = c("POSIXct", "POSIXt"), tzone = "", class = c("xts", 
"zoo"), index = structure(c(793929600, 794016000, 799200000, 
801878400, 804470400, 807148800), tzone = "", tclass = c("POSIXct", 
"POSIXt")), .Dim = c(6L, 1L))

Answer 1

1) index The object's class is "xts", not "data.frame". Use index (or time) to modify monthly.station6:

library(xts)
index(monthly.station6) <- as.yearmon(index(monthly.station6))

2) aggregate.zoo Another possibility is to use aggregate.zoo. That will return a "zoo" object so convert it back to "xts" :

library(xts)
as.xts(aggregate(monthly.station6, as.yearmon))

3) fortify.zoo Since the question mentions data.frame, if what you really wanted was a data.frame then the first statement after library will create a data.frame with a first column of Index and the second will perform the conversion to "yearmon":

library(xts)
DF <- fortify.zoo(monthly.station6)
transform(DF, Index = as.yearmon(Index))

Note: If you want just year then it cannot be an xts object but you could represent it as a data.frame. Using DF from (3):

transform(DF, Index = as.numeric(format(Index, "%Y")))