Introduction to Stata 8 - (GRIPS

15.3. Dates. Danish CPR numbers
Date variables [U] 15.5.2
Dates are numeric variables; the internal value is the number of days since 1 Jan 1960; dates
before that are negative.
Date formats [U] 15.5.3; [U] 27.2.3
Format specifications start with %d. Specifying %d only is equivalent to %dDlCY
displaying a date as 28sep2000. To display this date as 28.09.2000 specify the format
%dD.N.CY (D for day, N for numeric month, C for century, Y for two-digit year). Example:
format bdate %dD.N.CY
Reading date variables [U] 27.2.1
A date may be input as three variables: day, month, year and next transformed to a date
variable:
infix bd 1-2 bm 3-4 by 5-8 using c:\dokumenter\p1\datefile.txt
generate bdate = mdy(bm,bd,by) // sequence must be m d y
format bdate %dD.N.CY
Another option is to enter the date as a string (sbdate) and translate it to a date variable:
infix str10 sbdate 1-10 using c:\dokumenter\p1\datefile.txt
generate bdate = date(sbdate,"dmy") // "dmy" defines sequence
format bdate %dD.N.CY
The date function 'understands' most input formats: 17jan2001, 17/1/2001,
17.1.2001, 17 01 2001, but not 17012001. However todate, a user-written function,
handles this situation; find and download it by: findit todate.
In general: enter 4-digit years to avoid ambiguity on the century.
Calculations with dates [U] 27.2.4
To express the length of a time interval in years you must:
generate opage = (opdate-bdate)/365.25
You may extract day, month and year from a date variable (bdate):
generate bday = day(bdate)
gen bmonth = month(bdate)
gen byear = year(bdate)
On Danish CPR numbers: extracting key information
Sometimes you get date information as a CPR number in an ASCII file. You can read the the
CPR number as one string variable and the date information from the same columns:
infix str10 cprstr 1-10 bday 1-2 bmon 3-4 byear 5-6 ///
control 7-10 using c:\dokumenter\p1\datefile.txt
generate bdate = mdy(bmon,bday,byear)
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