Times and dates are handled by PSPP as single numbers. A time is an interval. PSPP measures times in seconds. Thus, the following intervals correspond with the numeric values given:
10 minutes 600
1 hour 3,600
1 day, 3 hours, 10 seconds 97,210
40 days 3,456,000
A date, on the other hand, is a particular instant in the past or the future. PSPP represents a date as a number of seconds since midnight preceding 14 Oct 1582. Because midnight preceding the dates given below correspond with the numeric PSPP dates given:
15 Oct 1582 86,400
4 Jul 1776 6,113,318,400
1 Jan 1900 10,010,390,400
1 Oct 1978 12,495,427,200
24 Aug 1995 13,028,601,600
Ordinary arithmetic operations on dates and times often produce sensible results. Adding a time to, or subtracting one from, a date produces a new date that much earlier or later. The difference of two dates yields the time between those dates. Adding two times produces the combined time. Multiplying a time by a scalar produces a time that many times longer. Since times and dates are just numbers, the ordinary addition and subtraction operators are employed for these purposes.
Adding two dates does not produce a useful result.
As the table shows, dates and times may have very large values. Thus, it is not a good idea to take powers of these values; also, the accuracy of some procedures may be affected. If necessary, convert times or dates in seconds to some other unit, like days or years, before performing analysis.