Three weeks ago I wrote about MATLAB's new spreadsheet import tool. Since then I've had a few conversations regarding using dates in MATLAB; dates are common as column headers or table data. The import tool will turn Excel dates into MATLAB datenums. A datenum in MATLAB is just a double that represents any date & time after midnight Jan 1, 0000 AD. MATLAB comes with several useful functions for handling these special case numbers, and in particular for displaying them.

The now, clock, and date functions will provide the current date & time as a datenum, datevec, or detestr, respectively. While a datenum is a single number representing the date and time, a datevec splits the year, month, day, hour, minute, and second components out in 1x6 vector. A datestr is the string representation of a date time. There are a ton formatting options available for how the date and time are displayed (see below).

currentTimeAndDate = now
currentTimeAndDateAsVector = clock
currentDateAsString = date

currentTimeAndDate =

   7.3484e+05


currentTimeAndDateAsVector =

         2011           11           28            8           50       20.486


currentDateAsString =

28-Nov-2011

As far as the data is concerned, these three date types are interchangeable using the conversion functions. You can use the datenum, datevec, and datestr functions to convert between the three types. Individual functions that perform calculations on dates usually prefer a particular format, for example addtodate looks for datenum types.

currentDateAsNumber = datenum(currentDateAsString)
currentDateAndDtimeAsNumber = datenum(currentTimeAndDate) %note the difference from above
currentDate = datestr(currentTimeAndDate)
currentDateAsVector = datevec(currentDateAsString)

currentDateAsNumber =

      734835


currentDateAndDtimeAsNumber =

   7.3484e+05


currentDate =

28-Nov-2011 08:50:20


currentDateAsVector =

        2011          11          28           0           0           0

The usage of dates I'm particularly interested in is with plots. Let's say we have some time-varying data and we want the x-axis to reflect those dates. If I just plot the data against the datenum, by default the x-labels will all be large, ugly numbers.

firstInMonths = repmat([2011 1 1 0 0 0],12,1);
firstInMonths(:,2) = 1:12;
bar(datenum(firstInMonths),rand(1,12)*10)

Thankfully, MATLAB comes with a simple function, datetick, for turning those numbers into strings:

datetick

Unfortunately, those tick labels aren't very pretty. Let's use a date format with the datetick function to specify how we want labels to look. For this function, we can use any date format recognized by the detestr function (see below).

datetick('x','mmm-yy')

Finally, for your reference, here is the help information for datestr, which lists how to formate a date string:

help datestr

 DATESTR String representation of date.
    S = DATESTR(V) converts one or more date vectors V to date strings S.
    Input V must be an M-by-6 matrix containing M full (six-element) date
    vectors. Each element of V must be a positive double-precision number.
    DATESTR returns a column vector of M date strings, where M is the total
    number of date vectors in V. 
 
    S = DATESTR(N) converts one or more serial date numbers N to date
    strings S. Input argument N can be a scalar, vector, or
    multidimensional array of positive double-precision numbers. DATESTR
    returns a column vector of M date strings, where M is the total number
    of date numbers in N. 
 
    S = DATESTR(D, F) converts one or more date vectors, serial date
    numbers, or date strings D into the same number of date strings S.
    Input argument F is a format number or string that determines the
    format of the date string output. Valid values for F are given in Table
    1, below. Input F may also contain a free-form date format string
    consisting of format tokens as shown in Table 2, below. 
 
    Date strings with 2-character years are interpreted to be within the
    100 years centered around the current year. 
 
    S = DATESTR(S1, F, P) converts date string S1 to date string S,
    applying format F to the output string, and using pivot year P as the
    starting year of the 100-year range in which a two-character year
    resides. The default pivot year is the current year minus 50 years.
    F = -1 uses the default format.
 
 	S = DATESTR(...,'local') returns the string in a localized format. The
 	default (which can be called with 'en_US') is US English. This argument 
 	must come last in the argument sequence.
 
 	Note:  The vectorized calling syntax can offer significant performance
 	improvement for large arrays.
 
 	Table 1: Standard MATLAB Date format definitions
 
    Number           String                   Example
    ===========================================================================
       0             'dd-mmm-yyyy HH:MM:SS'   01-Mar-2000 15:45:17 
       1             'dd-mmm-yyyy'            01-Mar-2000  
       2             'mm/dd/yy'               03/01/00     
       3             'mmm'                    Mar          
       4             'm'                      M            
       5             'mm'                     03            
       6             'mm/dd'                  03/01        
       7             'dd'                     01            
       8             'ddd'                    Wed          
       9             'd'                      W            
      10             'yyyy'                   2000         
      11             'yy'                     00           
      12             'mmmyy'                  Mar00        
      13             'HH:MM:SS'               15:45:17     
      14             'HH:MM:SS PM'             3:45:17 PM  
      15             'HH:MM'                  15:45        
      16             'HH:MM PM'                3:45 PM     
      17             'QQ-YY'                  Q1-96        
      18             'QQ'                     Q1           
      19             'dd/mm'                  01/03        
      20             'dd/mm/yy'               01/03/00     
      21             'mmm.dd,yyyy HH:MM:SS'   Mar.01,2000 15:45:17 
      22             'mmm.dd,yyyy'            Mar.01,2000  
      23             'mm/dd/yyyy'             03/01/2000 
      24             'dd/mm/yyyy'             01/03/2000 
      25             'yy/mm/dd'               00/03/01 
      26             'yyyy/mm/dd'             2000/03/01 
      27             'QQ-YYYY'                Q1-1996        
      28             'mmmyyyy'                Mar2000        
      29 (ISO 8601)  'yyyy-mm-dd'             2000-03-01
      30 (ISO 8601)  'yyyymmddTHHMMSS'        20000301T154517 
      31             'yyyy-mm-dd HH:MM:SS'    2000-03-01 15:45:17 
 
    Table 2: Free-form date format symbols
    
    Symbol  Interpretation of format symbol
    ===========================================================================
    yyyy    full year, e.g. 1990, 2000, 2002
    yy      partial year, e.g. 90, 00, 02
    mmmm    full name of the month, according to the calendar locale, e.g.
            "March", "April" in the UK and USA English locales. 
    mmm     first three letters of the month, according to the calendar 
            locale, e.g. "Mar", "Apr" in the UK and USA English locales. 
    mm      numeric month of year, padded with leading zeros, e.g. ../03/..
            or ../12/.. 
    m       capitalized first letter of the month, according to the
            calendar locale; for backwards compatibility. 
    dddd    full name of the weekday, according to the calendar locale, e.g.
            "Monday", "Tuesday", for the UK and USA calendar locales. 
    ddd     first three letters of the weekday, according to the calendar
            locale, e.g. "Mon", "Tue", for the UK and USA calendar locales. 
    dd      numeric day of the month, padded with leading zeros, e.g. 
            05/../.. or 20/../.. 
    d       capitalized first letter of the weekday; for backwards 
            compatibility
    HH      hour of the day, according to the time format. In case the time
            format AM | PM is set, HH does not pad with leading zeros. In 
            case AM | PM is not set, display the hour of the day, padded 
            with leading zeros. e.g 10:20 PM, which is equivalent to 22:20; 
            9:00 AM, which is equivalent to 09:00.
    MM      minutes of the hour, padded with leading zeros, e.g. 10:15, 
            10:05, 10:05 AM.
    SS      second of the minute, padded with leading zeros, e.g. 10:15:30,
            10:05:30, 10:05:30 AM. 
    FFF     milliseconds field, padded with leading zeros, e.g.
            10:15:30.015.
    PM      set the time format as time of morning or time of afternoon. AM 
            or PM is appended to the date string, as appropriate. 
 
    Examples:
 	DATESTR(now) returns '24-Jan-2003 11:58:15' for that particular date,
 	on an US English locale DATESTR(now,2) returns 01/24/03, the same as
 	for DATESTR(now,'mm/dd/yy') DATESTR(now,'dd.mm.yyyy') returns
 	24.01.2003 To convert a non-standard date form into a standard MATLAB
 	dateform, first convert the non-standard date form to a date number,
 	using DATENUM, for example, 
 	DATESTR(DATENUM('24.01.2003','dd.mm.yyyy'),2) returns 01/24/03.
 
 	See also DATE, DATENUM, DATEVEC, DATETICK.

    Reference page in Help browser
       doc datestr

If you're interested in using dates with MATLAB, here is a link to the date functions that are available.