Working with Time Series Data

SAS software represents dates as the number of days since a reference date. The reference date, or date zero, used for SAS date values is 1 January 1960. For example, 3 February 1960 is represented by SAS as 33. The SAS date for 17 October 1991 is 11612.

SAS software correctly represents dates from the year 1582 to the year 20,000.

Dates represented in this way are called SAS date values. Any numeric variable in a SAS data set whose values represent dates in this way is called a SAS date variable.

Representing dates as the number of days from a reference date makes it easy for the computer to store them and perform calendar calculations, but these numbers are not meaningful to users. However, you never have to use SAS date values directly, since SAS automatically converts between this internal representation and ordinary ways of expressing dates, provided that you indicate the format with which you want the date values to be displayed. (Formatting of date values is explained in the section Formatting Date and Datetime Values.)

SAS software informats, functions, and formats can process dates that are represented with two-digit year values. The century assumed for a two-digit year value can be controlled with the YEARCUTOFF= option in the OPTIONS statement. The YEARCUTOFF= system option controls how dates with two-digit year values are interpreted by specifying the first year of a 100-year span. The default value for the YEARCUTOFF= option is 1920. Thus by default the year '17' is interpreted as 2017, while the year '25' is interpreted as 1925. (See SAS Language Reference: Dictionary for more information about YEARCUTOFF=.)

SAS date values are written in a SAS program by placing the dates in single quotes followed by a D. The date is represented by the day of the month, the three letter abbreviation of the month name, and the year.

For example, SAS reads the value '17OCT1991'D the same as 11612, the SAS date value for 17 October 1991. Thus, the following SAS statements print DATE=11612:

data _null_;
  date = '17oct1991'd;
  put date=;
run;

The year value can be given with two or four digits, so '17OCT91'D is the same as '17OCT1991'D.

To represent both the time of day and the date, SAS uses datetime values. SAS datetime values represent the date and time as the number of seconds the time is from a reference time. The reference time, or time zero, used for SAS datetime values is midnight, 1 January 1960. Thus, for example, the SAS datetime value for 17 October 1991 at 2:45 in the afternoon is 1003329900.

To specify datetime constants in a SAS program, write the date and time in single quotes followed by DT. To write the date and time in a SAS datetime constant, write the date part using the same syntax as for date constants, and follow the date part with the hours, the minutes, and the seconds, separating the parts with colons. The seconds are optional.

For example, in a SAS program you would write 17 October 1991 at 2:45 in the afternoon as '17OCT91:14:45'DT. SAS reads this as 1003329900. Table 3.1 shows some other examples of datetime constants.

The SAS System also supports time values. SAS time values are just like datetime values, except that the date part is not given. To write a time value in a SAS program, write the time the same as for a datetime constant, but use T instead of DT. For example, 2:45:32 p.m. is written '14:45:32'T. Time values are represented by a number of seconds since midnight, so SAS reads '14:45:32'T as 53132.

SAS time values are not very useful for identifying time series, since usually both the date and the time of day are needed. Time values are not discussed further in this book.