time objects

Description Usage Arguments Details Examples

Converts an object representing a date (and if applicable a time) into an object of class POSIXct. Supported input classes are character, Date and POSIXt.

1	hsToPosix(datetime, keepTZ = is.null(tzone), tzone = NULL, lt = FALSE, ...)

`datetime`	object of class POSIXt or Date or character representing date (and time) information to be converted to class POSIXct.
`keepTZ`	if `TRUE` and if the given object is already of POSIX-type, the returned POSIXct object will be in the same time zone as the original object. Otherwise POSIX-objects will be returned in the time zone tzone.
`tzone`	time zone. Will be set to “UTC” if missing. UTC it the preferred time zone as it seems that only UTC prevents the POSIXt-classes from applying daylight-savings time.
`lt`	if TRUE a POSIXlt object is returned instead of a POSIXct object.
`...`	further arguments to be passed to as.POSIXct/as.POSIXlt, e.g. format, help for as.POSIXct/as.POSIXlt.

If datetime is already of class POSIXlt or POSIXct the time zone is preserved unless keepTZ is FALSE. If datetime is a character string it is expected to be in ISO format: “yyyy-mm-dd [HH:MM:SS]” where the time-part in brackets is optional.

# Start with a string representing a timestamp
datetime <- "2011-01-02 12:34:56"
  
# By default hsToPosix creates a POSIXct object:
ct <- hsToPosix(datetime)
class(ct) # "POSIXct" "POSIXt"
  
# You may decide to create a POSIXlt object instead:
lt <- hsToPosix(datetime, lt = TRUE)
class(lt) # "POSIXlt" "POSIXt"
  
# With a POSIXlt object you can access the different parts of the timestamp
sprintf("%d hours, %d minutes, %d seconds", lt$hour, lt$min, lt$sec)

# These are all available pieces of information 
# (isdst = is daylight savings time in effect)
sapply(attr(lt, "names"), function(name) try(lt[[name]]))
  
# You may use hsToPosix to convert between lt and ct
identical(hsToPosix(ct, lt = TRUE), lt)
identical(hsToPosix(lt, lt = FALSE), ct)
  
# The following time does not exist in CET/CEST but in UTC 
# as it is the time when daylight-savings time switched.
hsToPosix("2011-03-27 02:00:00") # "2011-03-27 02:00:00 UTC"
  
# Compare with as.POSIXct: between 02:00:00 and 02:59:59 the 
# time information gets lost and is only recognized again 
# from 03:00:00 on. Similar results with as.POSIXlt.
as.POSIXlt("2011-03-27 01:59:59") # "2011-03-27 01:59:59"
as.POSIXlt("2011-03-27 02:00:00") # "2011-03-27"
as.POSIXlt("2011-03-27 02:59:59") # "2011-03-27"
as.POSIXlt("2011-03-27 03:00:00") # "2011-03-27 03:00:00"  
  
# When loading data from an Access table it will be of class
# POSIXct:
#dat <- hsGetTable(xmdb(), "tbl_Hyd")
#class(dat$Zeitst) # "POSIXct" "POSIXt"
  
# In order to prevent R from considering daylight savings time
# we should convert to UTC time zone. But then we have to keep
# in mind that the indication "UTC" is not correct as the time
# stamps in fact represent the time zone "UTC+1"!
#head(dat$Zeitst) 
# "2011-08-23 00:00:00 CEST" "2011-08-23 00:01:00 CEST" ...
  
#head(hsToPosix(dat$Zeitst))  
# "2011-08-23 00:00:00 UTC" "2011-08-23 00:01:00 UTC" ...