Following are the three main System Variables:
This variable gives the current date as the number of days from October 14, 1582.
COMPUTE today = $jdate.
This can be used when you want to specify that a newly created variable (or some of its values) should be set to system missing.
compute miss = $sysmis.
compute miss1 = 1.
if missing(q1) or missing(q3) miss1 = $sysmis.
You can use this variable if you want to create an id variable that is part of your data set.
compute id = $casenum.