CLHS: Section 25.1.4.2

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## 25.1.4.2 Universal Time

Universal time is an absolute time represented as a single non-negative integer---the number of seconds since midnight, January 1, 1900 GMT (ignoring leap seconds). Thus the time 1 is 00:00:01 (that is, 12:00:01 a.m.) on January 1, 1900 GMT. Similarly, the time 2398291201 corresponds to time 00:00:01 on January 1, 1976 GMT. Recall that the year 1900 was not a leap year; for the purposes of Common Lisp, a year is a leap year if and only if its number is divisible by 4, except that years divisible by 100 are not leap years, except that years divisible by 400 are leap years. Therefore the year 2000 will be a leap year. Because universal time must be a non-negative integer, times before the base time of midnight, January 1, 1900 GMT cannot be processed by Common Lisp.

    decode-universal-time  get-universal-time
    encode-universal-time

Figure 25-6. Defined names involving time in Universal Time.

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