How to transform dates in Y-m format without days [duplicate]

Question

I have a data vector that looks like this:

dates<-c("2014-11", "2014-12", "2015-01", "2015-02", "2015-03", "2015-04")

I am trying to convert it into a recognizable date format, however no luck:

as.Date(dates,"%Y-%m")
[1] NA NA NA NA NA NA

I suspect that the problem lies in that that there is no day specified.

Any thoughs of how this can be solved?

Answer 1

If we need to convert to Date class, it needs a day. So, we can paste with one of the days of interest, say 1, and use as.Date

as.Date(paste0(dates, "-01"))

Answer 2

The zoo package has a nice interface to this, which allows storing of year-month data and a as.Date method to coerce to a Date object. For example:

library("zoo")
dates <- c("2014-11", "2014-12", "2015-01", "2015-02", "2015-03", "2015-04")

The function to convert the character vector or year-months into a yearmon is as.yearmon. The second argument is the format of the date parts in the individual strings. Here I use

• %Y for year with century
• %m for the month as a decimal
• Separated by literal -

.

yrmo <- as.yearmon(dates, "%Y-%m")

This gives

> yrmo
[1] "Nov 2014" "Dec 2014" "Jan 2015" "Feb 2015" "Mar 2015" "Apr 2015"

This is actually the default, so you can leave off the format part entirely, e.g. yrmo <- as.yearmon(dates)

To convert to a Date class object, the as.Date method is used

> as.Date(yrmo)
[1] "2014-11-01" "2014-12-01" "2015-01-01" "2015-02-01" "2015-03-01"
[6] "2015-04-01"

This method has a second argument frac which is specified allows you to state how far through the month you want each resulting Date element to be (how many days as a fraction of the length of the month in days)

> as.Date(yrmo, frac = 0.5)
[1] "2014-11-15" "2014-12-16" "2015-01-16" "2015-02-14" "2015-03-16"
[6] "2015-04-15"