Time is represented in four different ways in Common Lisp:
decoded time,
universal time,
internal time,
and seconds.
Decoded time and universal time are used primarily to represent calendar time,
and are precise only to one second.
Internal time is used primarily to represent measurements of computer
time (such as run time) and is precise to some implementation-dependent
fraction of a second called an internal time unit,
as specified by internal-time-units-per-second.
An internal time can be used
for either absolute and relative time measurements.
Both a universal time and a decoded time can be used
only for absolute time measurements.
In the case of one function, sleep,
time intervals are represented as a non-negative real number of seconds.
The next figure shows defined names relating to time.
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Figure 25.4: Defined names involving Time.
A decoded time is an ordered series of nine values that, taken together, represent a point in calendar time (ignoring leap seconds):
An integer between 0 and 59, inclusive.
An integer between 0 and 59, inclusive.
An integer between 0 and 23, inclusive.
An integer between 1 and 31, inclusive (the upper limit actually
depends on the month and year, of course).
An integer between 1 and 12, inclusive;
1 means January, 2 means February, and so on; 12 means December.
An integer indicating the year A.D. However, if this
integer
is between 0 and 99, the “obvious” year is used; more precisely,
that year is assumed that is equal to the
integer modulo 100 and
within fifty years of the current year (inclusive backwards
and exclusive forwards).
Thus, in the year 1978, year 28 is 1928
but year 27 is 2027. (Functions that return time in this format always return
a full year number.)
An integer between 0 and 6, inclusive;
0 means Monday, 1 means Tuesday, and so on; 6 means Sunday.
A generalized boolean that,
if true, indicates that daylight saving time is in effect.
A time zone.
The next figure shows defined names relating to decoded 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.
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Figure 25.6: Defined names involving time in Universal Time.
Internal time represents time as a single integer, in terms of an implementation-dependent unit called an internal time unit. Relative time is measured as a number of these units. Absolute time is relative to an arbitrary time base.
The next figure shows defined names related to internal time.
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Figure 25.7: Defined names involving time in Internal Time.
One function, sleep, takes its argument as a non-negative real number
of seconds. Informally, it may be useful to think of this as
a relative universal time, but it differs in one important way:
universal times are always non-negative integers, whereas the argument to
sleep can be any kind of non-negative real, in order to allow for
the possibility of fractional seconds.