This post is about a quirk in the way R handles dates.

I have a data set, rfm, which has a column named cohort, of class Date as shown below. I also have a vector of calendar dates, rather cleverly named dates, whose elements are also of class Date:

> class(rfm)
[1] "tbl_df"     "data.frame"
> class(rfm$cohort) [1] "Date" > class(dates) [1] "Date"  Both rfm$cohort and dates show the date of the first of the month over a certain time period, with the possibility that dates covers a more recent set of months. My problem is simple: I just want to see how many months there are between max(rfm$cohort) and max(dates). The lubridate package makes this easy with the interval() function, but the arguments of that function must be POSIXct, not Date objects, although this will not be readily apparent: > interval(max(rfm$cohort),max(dates))
[1] 2015-05-31 20:00:00 EDT--2015-06-30 20:00:00 EDT


See? No error message whatsoever. Let’s take a closer look:

> str(interval(max(rfm$cohort),max(dates))) Formal class 'Interval' [package "lubridate"] with 3 slots ..@ .Data: num 2592000 ..@ start: POSIXct[1:1], format: "2015-05-31 20:00:00" ..@ tzone: chr ""  Well, there’s no time zone, but other than that this seems to work. Except that it does not: > as.period(interval(max(rfm$cohort),max(dates)), months)
Error in while (any(start + est * per < end)) est[start + est * per <  :
missing value where TRUE/FALSE needed


This is not all that informative, but what if we recast the two arguments as POSIXct objects?

> as.period(interval(ymd(as.character(max(rfm$cohort))),ymd(as.character(max(dates)))), months) [1] "1m 0d 0H 0M 0S"  This worked! But why? > str(interval(ymd(as.character(max(rfm$cohort))),ymd(as.character(max(dates)))))
Formal class 'Interval' [package "lubridate"] with 3 slots
..@ .Data: num 2592000
..@ start: POSIXct[1:1], format: "2015-06-01"
..@ tzone: chr "UTC"


Seems like two things are different: we now have a time zone, UTC – which stands for Universal Time, Coordinated, which is another name for the old-school GMT – and the start point is in this date format, as opposed to the time stamp above.

Now: do we really need a chained call to two functions, ymd() and as.character()? Wouldn’t as.POSIXct() suffice? Let’s try:

> as.period(interval(as.POSIXct(max(rfm$cohort), tz = 'GMT'),as.POSIXct(max(dates), tz = 'GMT')), months) Error in while (any(start + est * per < end)) est[start + est * per < : missing value where TRUE/FALSE needed  Nope. But why? If you look up ?as.POSIXct you will see that tz = is the argument that sets the time zone, so this should work. As it turns out, lubridate wants you to set the time zone for the interval, not separately for its ends. Like this: > as.period(interval(as.POSIXct(max(rfm$cohort)),as.POSIXct(max(dates)), tz = 'GMT'), months)
[1] "1m 0d 0H 0M 0S"


I have no idea why this is so, and asking StackOverflow hasn’t turned up anything so far.