On Thu, 24 Jul 2008, DCC wrote:
I frequently work with hourly data in GMT format, and ensure that any data I read into R are expressed in GMT through as.POSIXct(mydate, tz = "GMT").
That makes no sense. POSIXct times are absolute times, not in any timezone. You can display them in any timezone you like, e.g. by the format method.
If you have a character representation of a time to convert by as.POSIXct, you do need to tell R the timezone used in the character representation.
I am interested in processing air pollution data in GMT where where peaks in emissions (say from road traffic sources) tend to occur at the same LOCAL time each day. I am interested in calculating mean concentrations by hour of day i.e. a diurnal profile, consisting of 24 hourly means. The profile would be clearer if all hours were in local time e.g. the daylight saving hours were shifted by +1 hour. This would ensure that the summer and winter profiles match up with those in emissions. I've looked though the various R documents/forum, but can't seem to identify the easiest approach to this. Any suggestions?
I think you want
T <- Sys.time() as.POSIXlt(T, tz="Europe/London")$hour
which gives you the hour in the local timezone of Leeds.
Many thanks. David
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