SPSS® 12.0 Command Syntax Reference

Precautions with Date and Time Variables
Date and Time 69
Dates and times are represented internally as seconds. The numbers for dates are very large,
and arithmetic overflows can result. For example, dates in the 20th century are on the order
of 10 to the 10th power (11 digits). For that reason, a few precautions are in order:
• Some machine environments cannot accommodate the computation of higher powers of
date and time variables. For example, computations higher than the sixth power may
cause overflows on some machines.
• The magnitude of the values may cause inaccuracies in some statistical procedures. It is
advisable to subtract a fixed date if you want to keep seconds as the unit, or to convert
days using the XDATE.TDAYS function. REGRESSION, CORRELATIONS, ANOVA, and
ONEWAY use an adaptive centering method, so their accuracy will not be affected.
• LIST, REPORT, and TABLES are the only procedures that display values in date and time formats.
The PRINT and WRITE transformation commands can also display and write date and
time formats. However, some summary variables in REPORT and calculated variables in
TABLES display in F format, regardless of the print formats of variables used as arguments.
• All other procedures use F format in all cases. The default width and number of decimal
places is taken from the print format, but the format type is ignored. For example, in a
frequency table, the date 1/09/57 with a print format of DATE9 will be displayed as
11830147200, not 01-SEP-57.
• Changing the print format in no way alters the values that are stored. For example, if you
assign a print format of DATE9 for a variable read with DATETIME format, the time of day
will not display but continues to be part of the value. This means that seemingly identical
values can be displayed as separate entries within procedures.