as.POSIXct gives an unexpected timezone

Question

I\'m trying to convert a yearmon date (from the zoo package) to a POSIXct in the UTC timezone. This is what I tried to do:

> as.POSIXct(as.yearmon(\"2010         


        

Answer 1

In the help page ?as.POSIXct, for the tz argument it says

A timezone specification to be used for the conversion, if one is required. System-specific (see time zones), but ‘""’ is the current timezone, and ‘"GMT"’ is UTC (Universal Time, Coordinated).

Does as.POSIXct(as.yearmon("2010-01-01"), tz="GMT") work for you?

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After more perusal of the documentation, in the details section we see:

Dates without times are treated as being at midnight UTC.

So in your example, the tz argument is ignored. If you use as.POSIXlt it is easier to see what happens with the timezone. The following should all give the same answer, with UTC as the timezone.

unclass(as.POSIXlt(as.yearmon("2010-01-01")))
unclass(as.POSIXlt(as.yearmon("2010-01-01"), tz = "UTC"))
unclass(as.POSIXlt(as.yearmon("2010-01-01"), tz = "GMT"))
unclass(as.POSIXlt(as.yearmon("2010-01-01"), tz = "CET"))

In fact, since you are using as.yearmon (which strips the time out) you will never get to set the timezone. Compare, e.g.,

unclass(as.POSIXlt(as.yearmon("2010-01-01 12:00:00"), tz = "CET"))
unclass(as.POSIXlt("2010-01-01 12:00:00", tz = "CET"))