

   CCrreeaattee aa CChhrroonnoollooggiiccaall OObbjjeecctt

        chron(dates, times, format = c("m/d/y","h:m:s"), out.format, origin)

   AArrgguummeennttss::

      dates: character or numeric vector specifying dates.  If
             character, `dates' are assumed to be in one of the
             date formats below; if numeric, `dates' are
             assumed to be Julian dates, i.e., number of days
             since `origin'.

      times: optional character or numeric vector specifying
             times of day.  If character, `times' are assumed
             to be in one of the time formats below; if
             numeric, `times' are assumed to be fractions of a
             day.

     format: vector or list specifying the input format of the
             input.  The format can be either strings specify-
             ing one of the recognized formats below or a list
             of user-supplied functions to convert dates from
             character into Julian dates and vice versa.

             The dates format can be any permutation of the
             characters "d", "m", or "y" delimited by a separa-
             tor (possibly null), e.g., "m/d/y", "d-m-y",
             "ymd", are all valid; the format can also be per-
             mutations of the words "day" "month" and "year",
             which produces the month name, e.g., "month day
             year" produces "April 20 1992", "day mon year"
             produces "20 Apr 1992".

             The times format can be any permutation of "h",
             "m", and "s" separated by any one non-special
             character.  The default is "h:m:s".

   out.format: vector or list specifying date and time format
             for printing and output. Default is same as `for-
             mat'.

     origin: a vector specifying the date with respect to which
             Julian dates are computed. Default is `c(month =
             1, day = 1, year = 1970)'; you may set the option
             `chron.origin' to specify your own default, e.g.,
             `options(chron.origin = c(month=1, day=1,
             year=1990))'.

   VVaalluuee::

        An object of class `times' if only `times' were speci-
        fied, `dates' if only `dates', or `chron' if both
        `dates = ' and `times = ' were supplied.  All these
        inherit from class `times'.

        These objects represent dates and times of day, and
        allow the following arithmetic and summaries:  subtrac-
        tion `d1-d2', constant addition `d1+constants', all
        logical comparisons, summaries `min()', `max()', and
        `range()' (which drop NAs by default); constants spec-
        ify days (fractions are converted to time-of-day, e.g.,
        2.5 represents 2 days and 12 hours).  Operations such
        as sorting, differencing, etc., are automatically han-
        dled.

        There are methods for `as.character()', `as.numeric()',
        `cut()', `is.na()', `print()', `summary()', `plot()',
        `lines()', `lag()', and the usual subsetting functions
        `[', `[<-'.  The functions `days()', `months()', `quar-
        ters()', `years()', `weeks()', `weekdays()', `hours()',
        `minutes()', and `seconds()' take any `chron' object as
        input and extract the corresponding time interval.
        `cut()' is used to create ordered factors from `chron'
        objects.  Chronological objects may be used with the
        modeling software.

        The current implementation of `chron' objects does not
        handle time zones nor daylight savings time.

   SSeeee AAllssoo::

        dates, times, cut.dates, seq.dates

   EExxaammpplleess::

        dts <- dates(c("02/27/92", "02/27/92", "01/14/92",
                       "02/28/92", "02/01/92"))
        dts
        # [1] 02/27/92 02/27/92 01/14/92 02/28/92 02/01/92
        tms <- times(c("23:03:20", "22:29:56", "01:03:30",
                       "18:21:03", "16:56:26"))
        tms
        # [1] 23:03:20 22:29:56 01:03:30 18:21:03 16:56:26
        x <- chron(dates = dts, times = tms)
        x
        # [1] (02/27/92 23:03:19) (02/27/92 22:29:56) (01/14/92 01:03:30)
        # [4] (02/28/92 18:21:03) (02/01/92 16:56:26)

        # We can add or subtract scalars (representing days) to dates or
        # chron objects:
        c(dts[1], dts[1] + 10)
        # [1] 02/27/92 03/08/92
        dts[1] - 31
        # [1] 01/27/92

        # We can substract dates which results in a times object that
        # represents days between the operands:
        dts[1] - dts[3]
        # Time in days:
        # [1] 44

        # Logical comparisons work as expected:
        dts[dts > "01/25/92"]
        # [1] 02/27/92 02/27/92 02/28/92 02/01/92
        dts > dts[3]
        # [1]  TRUE  TRUE FALSE  TRUE  TRUE

        # Summary operations which are sensible are permitted and work as
        # expected:
        range(dts)
        # [1] 01/14/92 02/28/92
        diff(x)
        # Time in days:
        # [1]  -0.02319444 -44.89335648  45.72052083 -27.05876157
        sort(dts)[1:3]
        # [1] 01/14/92 02/01/92 02/27/92

