Manipulate timezone

#
# prepare data according to system timezone. "Asia/Tokyo" in most cases.
#
bp.day <- merge(as.xts(as.vector(bp.xts[,1]),as.Date(index(bp.xts),tz=tzone(bp.xts))),as.vector(bp.xts[,2]))
colnames(bp.day)[1] <- "high"
colnames(bp.day)[2] <- "low"
#
# prepare timezone 2 hours behind "Asia/Tokyo".
#
bp.bangkok <- merge(as.xts(as.vector(bp.xts[,1]),as.Date(index(bp.xts),tz="Asia/Bangkok")),as.vector(bp.xts[,2]))
colnames(bp.bangkok)[1] <- "high"
colnames(bp.bangkok)[2] <- "low"
apply.weekly(bp.bangkok,mean)

• Set timezone to other than Tokyo, as blood pressure may be measured after the mid night.
• Setting time zone as "Bangkok" allows samples taken from 00:00 to 02:00 to be taken into the day before.

Calculate day dependent data with less workload - 1 set languge locale

In order to calculate day dependent data with less workload, I like to automate processes. The first step is to judge the first day of each belongs to which day of the week. weekday function provides this function.

Currently R's language setting is set to JA. This may be

> weekdays(as.Date("2016-01-01"),abbreviate = TRUE)
[1] "金"
> Sys.getlocale("LC_MESSAGES")
[1] "ja_JP.UTF-8"

Change LC_MESSAGES to US. But weekdays function still return the value in Japanese.

> Sys.setlocale("LC_MESSAGES",'en_US')
[1] "en_US"
> weekdays(as.Date("2016-01-01"),abbreviate = TRUE)
[1] "金"

Investigate Sys.setlocale and found that there are other classes than LC_MESSAGES. I have no problem with US English.

> Sys.setlocale
function (category = "LC_ALL", locale = "") 
{
    category <- match(category, c("LC_ALL", "LC_COLLATE", "LC_CTYPE", 
        "LC_MONETARY", "LC_NUMERIC", "LC_TIME", "LC_MESSAGES", 
        "LC_PAPER", "LC_MEASUREMENT"))
    if (is.na(category)) 
        stop("invalid 'category' argument")
    .Internal(Sys.setlocale(category, locale))
}
<bytecode: 0x102cfdee0>
<environment: namespace:base>

Set LC_ALL locale, instead of LC_MESSAGES to US English.

> Sys.setlocale("LC_ALL",'en_US')
[1] "en_US/en_US/en_US/C/en_US/en_US"

Then return is in US English.

> weekdays(as.Date("2016-01-01"),abbreviate = TRUE)
[1] "Fri"
>