How do I convert correctly timezones

Question

I am using the fasttime package for its fastPOSIXct function that can read character datetimes very efficiently. My problem is that it can only read character datet

Answer 1

It's a daylight savings issue: http://www.timeanddate.com/time/dst/2010a.html

In 2010 it began on the 14th March in Canada, but not until the 28th March in the UK.

You can use POSIXlt objects to modify timezones directly:

lt <- as.POSIXlt(as.POSIXct("2010-03-15 12:37:17.223",tz="GMT"))
attr(lt,"tzone") <- "America/Montreal"
as.POSIXct(lt)
[1] "2010-03-15 12:37:17 EDT"

Or you could use format to convert to a string and set the timezone in a call to as.POSIXct. You can therefore modify forceTZ:

forceTZ <- function(x,tz)
{
  return(as.POSIXct(format(x),tz=tz))
}


forceTZ(as.POSIXct("2010-03-15 12:37:17.223",tz="GMT"),"America/Montreal")
[1] "2010-03-15 12:37:17 EDT"