SPSS® 12.0 Command Syntax Reference

Date and Time 61
• A time interval is a floating-point number representing the number of seconds in a time
period, for example, an hour, minute, or day. For example, the value representing 5.5 days
is 475,200; the value representing the time interval 14:08:17 is 50,897.
• QYR, MOYR, and WKYR variables are stored as midnight of the first day of the respective
quarter, month, and week of the year. Therefore, 1 Q 90, 1/90, and 1 WK 90 are all equivalents
of January 1, 1990 0:0:00. See “Date and Time Functions” on p. 62 for information
on how to determine the quarter, month, or week of a year for a certain date.
• WKDAY variables are stored as 1 to 7, and MONTH variables as 1 to 12. For information on
how to determine the day of the week or the month of the year for a certain date, see “Date
and Time Functions” on p. 62.
• Both dates and time intervals can be used in arithmetic expressions. The results are stored
as the number of seconds or days (see Table 8).
• Do not mix time variables (TIME and DTIME) with date variables (DATE, ADATE, EDATE,
and so on) in computations. Since date variables have an implicit time value of 00:00:00,
calculations involving time values that are not multiples of a whole day (for example, 24
hours, 0 minutes, 0 seconds) will yield unreliable results.
• Mixing a DATETIME variable with a date variable may yield an unreliable result.
Operations involving date variables are accurate only to the days. To avoid possible
misinterpretation, use the DTIME format and ignore the hours and minutes portion of the
resulting value.
You can perform virtually any arithmetic operation with them. Of course, not all of these
operations are particularly useful. You can calculate the number of days between two dates
by subtracting one date from the other—but adding two dates does not produce a very meaningful
result.
By default, any new numeric variables you compute are displayed in F format. In the case
of calculations involving time and date variables, this means that the default output is expressed
as a number of seconds or days. Use the FORMATS (or PRINT FORMATS) command
to specify an appropriate format for the computed variable. Table 8 shows the recommended
output formats for some of the calculations possible with date and time variables.
Table 8 Recommended output formats for date and time calculations
Arithmetic operation Result Recommended output format
time ± time *
time TIME, DTIME
date – date †
time DTIME
DATETIME – DATETIME time TIME, DTIME
DATETIME ± time date DATETIME
* Including TIME and DTIME formats.
† Including DATE, ADATE, EDATE, JDATE, and SDATE formats.