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Stata 5: How do I make date variables?

You can record dates anyway you want, but there is only one technique that Stata understands, called an elapsed date. An elapsed date is the number of days from January 1, 1960. In this format,

                        0    means    January 1, 1960
                        1             January 2, 1960
                       -1             December 31, 1959
                      365             December 31, 1960

Stata provides functions to convert dates into elapsed dates, formats to print elapsed dates in understandable forms, and other functions to manipulate elapsed dates.

Conversion into elapsed dates

Two functions are provided: mdy() and date().

mdy() takes three numeric arguments — month, day, year — and returns the corresponding elapsed date. For the following data,

                    month   day    year
                        7    11    1948
                        1    21    1952
                       11     2    1994

gen edate = mdy(month, day, year)

will produce the following data:

                    month   day    year   edate
                        7    11    1948   -4191
                        1    21    1952   -2902
                       11     2    1994   12724

date(), on the other hand, takes two string arguments: the first is the date to be converted, and the second informs date() of the order of the month, day, and year. date() returns the corresponding elapsed date. For the following data,

                 datestr1           datestr2 
                7-11-1948          11-7-1948
                11.2.1994          2.11.1994
            Sept11,  1994         11Sept1994
        November 13, 1995   12 November 1995

        gen edate1 = date(datestr1, "mdy")
        gen edate2 = date(datestr2, "dmy")

will create two variables, edate1 and edate2, that contain the same information :

                 datestr1           datestr2     edate1    edate2 
                7-11-1948          11-7-1948      -4191     -4191
                11.2.1994          2.11.1994      12724     12724 
            Sept11,  1994         11Sept1994      12672     12672  
        November 13, 1995   12 November 1995      13100     13100

Displaying dates

Once converted to the elapsed date format, you can assign Stata's %d format to the variable so that the dates are readable. Several variations are possible for displaying dates. For the following data,

                    edate
                    -4191
                    12724
                    12672
                    13100

format edate %d

will change the display format to

                    edate
                  11jul48
                   2nov94
                  11sep94
                  13nov95 

format edate %dM_d,_CY

will change the display format to

                    edate
         January 11, 1948
         November 2, 1994
       September 11, 1994
        November 13, 1995 

Several other formats are possible. For online help, type help dates in Stata, or see [U] 30 Commands for dealing with dates.