parse_date_time: User friendly date-time parsing functions
In lubridate: Make Dealing with Dates a Little Easier

Description Usage Arguments Details Value Note See Also Examples

parse_date_time() parses an input vector into POSIXct date-time object. It differs from base::strptime() in two respects. First, it allows specification of the order in which the formats occur without the need to include separators and the % prefix. Such a formatting argument is referred to as "order". Second, it allows the user to specify several format-orders to handle heterogeneous date-time character representations.

parse_date_time2() is a fast C parser of numeric orders.

fast_strptime() is a fast C parser of numeric formats only that accepts explicit format arguments, just like base::strptime().

parse_date_time(
  x,
  orders,
  tz = "UTC",
  truncated = 0,
  quiet = FALSE,
  locale = Sys.getlocale("LC_TIME"),
  select_formats = .select_formats,
  exact = FALSE,
  train = TRUE,
  drop = FALSE
)

parse_date_time2(
  x,
  orders,
  tz = "UTC",
  exact = FALSE,
  lt = FALSE,
  cutoff_2000 = 68L
)

fast_strptime(x, format, tz = "UTC", lt = TRUE, cutoff_2000 = 68L)

`x`	a character or numeric vector of dates
`orders`	a character vector of date-time formats. Each order string is a series of formatting characters as listed in `base::strptime()` but might not include the `"%"` prefix. For example, "ymd" will match all the possible dates in year, month, day order. Formatting orders might include arbitrary separators. These are discarded. See details for the implemented formats. If multiple order strings are supplied, they are applied in turn for `parse_date_time2()` and `fast_strptime()`. For `parse_date_time()` the order of applied formats is determined by `select_formats` parameter.
`tz`	a character string that specifies the time zone with which to parse the dates
`truncated`	integer, number of formats that can be missing. The most common type of irregularity in date-time data is the truncation due to rounding or unavailability of the time stamp. If the `truncated` parameter is non-zero `parse_date_time()` also checks for truncated formats. For example, if the format order is "ymdHMS" and `truncated = 3`, `parse_date_time()` will correctly parse incomplete date-times like `2012-06-01 12:23`, `2012-06-01 12` and `2012-06-01`. NOTE: The `ymd()` family of functions is based on `base::strptime()` which currently fails to parse `%Y-%m` formats.
`quiet`	logical. If `TRUE`, progress messages are not printed, and `No formats found` error is suppressed and the function simply returns a vector of NAs. This mirrors the behavior of base R functions `base::strptime()` and `base::as.POSIXct()`.
`locale`	locale to be used, see locales. On Linux systems you can use `system("locale -a")` to list all the installed locales.
`select_formats`	A function to select actual formats for parsing from a set of formats which matched a training subset of `x`. It receives a named integer vector and returns a character vector of selected formats. Names of the input vector are formats (not orders) that matched the training set. Numeric values are the number of dates (in the training set) that matched the corresponding format. You should use this argument if the default selection method fails to select the formats in the right order. By default the formats with most formatting tokens (`%`) are selected and `%Y` counts as 2.5 tokens (so that it has a priority over `%y%m`). See examples.
`exact`	logical. If `TRUE`, the `orders` parameter is interpreted as an exact `base::strptime()` format and no training or guessing are performed (i.e. `train`, `drop` parameters are irrelevant).
`train`	logical, default `TRUE`. Whether to train formats on a subset of the input vector. The result of this is that supplied orders are sorted according to performance on this training set, which commonly results in increased performance. Please note that even when `train = FALSE` (and `exact = FALSE`) guessing of the actual formats is still performed on a pseudo-random subset of the original input vector. This might result in `All formats failed to parse` error. See notes below.
`drop`	logical, default `FALSE`. Whether to drop formats that didn't match on the training set. If `FALSE`, unmatched on the training set formats are tried as a last resort at the end of the parsing queue. Applies only when `train = TRUE`. Setting this parameter to `TRUE` might slightly speed up parsing in situations involving many formats. Prior to v1.7.0 this parameter was implicitly `TRUE`, which resulted in occasional surprising behavior when rare patterns where not present in the training set.
`lt`	logical. If `TRUE`, returned object is of class POSIXlt, and POSIXct otherwise. For compatibility with `base::strptime()` the default is `TRUE` for `fast_strptime()` and `FALSE` for `parse_date_time2()`.
`cutoff_2000`	integer. For `y` format, two-digit numbers smaller or equal to `cutoff_2000` are parsed as though starting with `20`, otherwise parsed as though starting with `19`. Available only for functions relying on `lubridate`s internal parser.
`format`	a vector of formats. If multiple formats supplied they are applied in turn till success. The formats should include all the separators and each format letter must be prefixed with %, just as in the format argument of `base::strptime()`.

When several format-orders are specified, parse_date_time() selects (guesses) format-orders based on a training subset of the input strings. After guessing the formats are ordered according to the performance on the training set and applied recursively on the entire input vector. You can disable training with train = FALSE.

parse_date_time(), and all derived functions, such as ymd_hms(), ymd(), etc., will drop into fast_strptime() instead of base::strptime() whenever the guessed from the input data formats are all numeric.

The list below contains formats recognized by lubridate. For numeric formats leading 0s are optional. As compared to base::strptime(), some of the formats are new or have been extended for efficiency reasons. These formats are marked with "(*)" below. Fast parsers parse_date_time2() and fast_strptime() accept only formats marked with "(!)".

a

Abbreviated weekday name in the current locale. (Also matches full name)

A

Full weekday name in the current locale. (Also matches abbreviated name).

You don't need to specify a and A formats explicitly. Wday is automatically handled if preproc_wday = TRUE

b (!)

Abbreviated or full month name in the current locale. The C parser currently understands only English month names.

B (!)

Same as b.

d (!)

Day of the month as decimal number (01–31 or 0–31)

H (!)

Hours as decimal number (00–24 or 0–24).

I (!)

Hours as decimal number (01–12 or 1–12).

j

Day of year as decimal number (001–366 or 1–366).

q (!*)

Quarter (1–4). The quarter month is added to the parsed month if m element is present.

m (!*)

Month as decimal number (01–12 or 1–12). For parse_date_time also matches abbreviated and full months names as b and B formats. C parser understands only English month names.

M (!)

Minute as decimal number (00–59 or 0–59).

p (!)

AM/PM indicator in the locale. Commonly used in conjunction with I and not with H. But lubridate's C parser accepts H format as long as hour is not greater than 12. C parser understands only English locale AM/PM indicator.

S (!)

Second as decimal number (00–61 or 0–61), allowing for up to two leap-seconds (but POSIX-compliant implementations will ignore leap seconds).

OS

Fractional second.

U

Week of the year as decimal number (00–53 or 0–53) using Sunday as the first day 1 of the week (and typically with the first Sunday of the year as day 1 of week 1). The US convention.

w

Weekday as decimal number (0–6, Sunday is 0).

W

Week of the year as decimal number (00–53 or 0–53) using Monday as the first day of week (and typically with the first Monday of the year as day 1 of week 1). The UK convention.

y (!*)

Year without century (00–99 or 0–99). In parse_date_time() also matches year with century (Y format).

Y (!)

Year with century.

z (!*)

ISO8601 signed offset in hours and minutes from UTC. For example -0800, -08:00 or -08, all represent 8 hours behind UTC. This format also matches the Z (Zulu) UTC indicator. Because base::strptime() doesn't fully support ISO8601 this format is implemented as an union of 4 formats: Ou (Z), Oz (-0800), OO (-08:00) and Oo (-08). You can use these formats as any other but it is rarely necessary. parse_date_time2() and fast_strptime() support all of these formats.

Om (!*)

Matches numeric month and English alphabetic months (Both, long and abbreviated forms).

Op (!*)

Matches AM/PM English indicator.

r (*)

Matches Ip and H orders.

R (*)

Matches HM andIMp orders.

T (*)

Matches IMSp, HMS, and HMOS orders.

a vector of POSIXct date-time objects

parse_date_time() (and the derivatives ymd(), ymd_hms(), etc.) relies on a sparse guesser that takes at most 501 elements from the supplied character vector in order to identify appropriate formats from the supplied orders. If you get the error All formats failed to parse and you are confident that your vector contains valid dates, you should either set exact argument to TRUE or use functions that don't perform format guessing (fast_strptime(), parse_date_time2() or base::strptime()).

For performance reasons, when timezone is not UTC, parse_date_time2() and fast_strptime() perform no validity checks for daylight savings time. Thus, if your input string contains an invalid date time which falls into DST gap and lt = TRUE you will get an POSIXlt object with a non-existent time. If lt = FALSE your time instant will be adjusted to a valid time by adding an hour. See examples. If you want to get NA for invalid date-times use fit_to_timeline() explicitly.

base::strptime(), ymd(), ymd_hms()

## ** orders are much easier to write **
x <- c("09-01-01", "09-01-02", "09-01-03")
parse_date_time(x, "ymd")
parse_date_time(x, "y m d")
parse_date_time(x, "%y%m%d")
#  "2009-01-01 UTC" "2009-01-02 UTC" "2009-01-03 UTC"

## ** heterogeneous date-times **
x <- c("09-01-01", "090102", "09-01 03", "09-01-03 12:02")
parse_date_time(x, c("ymd", "ymd HM"))

## ** different ymd orders **
x <- c("2009-01-01", "02022010", "02-02-2010")
parse_date_time(x, c("dmY", "ymd"))
##  "2009-01-01 UTC" "2010-02-02 UTC" "2010-02-02 UTC"

## ** truncated time-dates **
x <- c("2011-12-31 12:59:59", "2010-01-01 12:11", "2010-01-01 12", "2010-01-01")
parse_date_time(x, "Ymd HMS", truncated = 3)

## ** specifying exact formats and avoiding training and guessing **
parse_date_time(x, c("%m-%d-%y", "%m%d%y", "%m-%d-%y %H:%M"), exact = TRUE)
parse_date_time(c('12/17/1996 04:00:00','4/18/1950 0130'),
                c('%m/%d/%Y %I:%M:%S','%m/%d/%Y %H%M'), exact = TRUE)

## ** quarters and partial dates **
parse_date_time(c("2016.2", "2016-04"), orders = "Yq")
parse_date_time(c("2016", "2016-04"), orders = c("Y", "Ym"))

## ** fast parsing **
## Not run: 
  options(digits.secs = 3)
  ## random times between 1400 and 3000
  tt <- as.character(.POSIXct(runif(1000, -17987443200, 32503680000)))
  tt <- rep.int(tt, 1000)

  system.time(out <- as.POSIXct(tt, tz = "UTC"))
  system.time(out1 <- ymd_hms(tt)) # constant overhead on long vectors
  system.time(out2 <- parse_date_time2(tt, "YmdHMOS"))
  system.time(out3 <- fast_strptime(tt, "%Y-%m-%d %H:%M:%OS"))

  all.equal(out, out1)
  all.equal(out, out2)
  all.equal(out, out3)

## End(Not run)

## ** how to use `select_formats` argument **
## By default %Y has precedence:
parse_date_time(c("27-09-13", "27-09-2013"), "dmy")

## to give priority to %y format, define your own select_format function:

my_select <-   function(trained, drop=FALSE, ...){
   n_fmts <- nchar(gsub("[^%]", "", names(trained))) + grepl("%y", names(trained))*1.5
   names(trained[ which.max(n_fmts) ])
}

parse_date_time(c("27-09-13", "27-09-2013"), "dmy", select_formats = my_select)

## ** invalid times with "fast" parsing **
parse_date_time("2010-03-14 02:05:06",  "YmdHMS", tz = "America/New_York")
parse_date_time2("2010-03-14 02:05:06",  "YmdHMS", tz = "America/New_York")
parse_date_time2("2010-03-14 02:05:06",  "YmdHMS", tz = "America/New_York", lt = TRUE)