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timestamp doc minor improvements

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* doc/lispref/os.texi (Time of Day, Time Conversion)
(Time Calculations): Fix some confusion about decoded times,
timestamps, and time values.  Exclude floating-point infinities
and NaNs from timestamps, as the code doesn’t always follow
IEEE-754 rules for them and whatever the code does, doesn’t matter
for timestamps anyway.
This commit is contained in:
Paul Eggert 2022-05-05 08:56:19 -07:00
parent d04acc1946
commit 8fce81897d
1 changed files with 11 additions and 13 deletions
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24
doc/lispref/os.texi
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@ -1371,7 +1371,7 @@ may change as higher-resolution clocks become available.
Function arguments, e.g., the @var{time} argument to
@code{format-time-string}, accept a more-general @dfn{time value}
format, which can be a Lisp timestamp, @code{nil} for the current
time, a single floating-point number for seconds, or a list
time, a finite floating-point number for seconds, or a list
@code{(@var{high} @var{low} @var{micro})} or @code{(@var{high}
@var{low})} that is a truncated list timestamp with missing elements
taken to be zero.
@ -1558,13 +1558,13 @@ Although an omitted or @code{nil} @var{form} currently acts like
@code{list}, this is planned to change in a future Emacs version, so
callers requiring list timestamps should pass @code{list} explicitly.
If @var{time} is infinite or a NaN, this function signals an error.
If @var{time} is not a time value, this function signals an error.
Otherwise, if @var{time} cannot be represented exactly, conversion
truncates it toward minus infinity. When @var{form} is @code{t},
conversion is always exact so no truncation occurs, and the returned
clock resolution is no less than that of @var{time}. By way of
contrast, @code{float-time} can convert any Lisp time value without
signaling an error, although the result might not be exact.
contrast, although @code{float-time} can also convert any time value
without signaling an error, the result might not be exact.
@xref{Time of Day}.
For efficiency this function might return a value that is @code{eq} to
@ -1652,7 +1652,7 @@ a particular form should specify @var{form}.
@var{dow} and @var{utcoff}, and its @var{second} is an integer between
0 and 59 inclusive.
To access (or alter) the elements in the time value, the
To access (or alter) the elements in the calendrical information, the
@code{decoded-time-second}, @code{decoded-time-minute},
@code{decoded-time-hour}, @code{decoded-time-day},
@code{decoded-time-month}, @code{decoded-time-year},
@ -1755,7 +1755,7 @@ at the 15th of the month when adding months. Alternatively, you can use the
@cindex formatting time values
These functions convert time values to text in a string, and vice versa.
Time values include @code{nil}, numbers, and Lisp timestamps
Time values include @code{nil}, finite numbers, and Lisp timestamps
(@pxref{Time of Day}).
@defun date-to-time string
@ -2067,25 +2067,23 @@ interactively, it prints the duration in the echo area.
These functions perform calendrical computations using time values
(@pxref{Time of Day}). As with any time value, a value of
@code{nil} for any of their
time-value arguments stands for the current system time, and a single
time-value arguments stands for the current system time, and a finite
number stands for the number of seconds since the epoch.
@defun time-less-p t1 t2
This returns @code{t} if time value @var{t1} is less than time value
This returns @code{t} if the time value @var{t1} is less than the time value
@var{t2}.
The result is @code{nil} if either argument is a NaN.
@end defun
@defun time-equal-p t1 t2
This returns @code{t} if @var{t1} and @var{t2} are equal time values.
The result is @code{nil} if either argument is a NaN.
This returns @code{t} if the two time values @var{t1} and @var{t2} are
equal.
@end defun
@defun time-subtract t1 t2
This returns the time difference @var{t1} @minus{} @var{t2} between
two time values, as a Lisp time value. The result is exact and its clock
two time values, as a Lisp timestamp. The result is exact and its clock
resolution is no worse than the worse of its two arguments' resolutions.
The result is floating-point only if it is infinite or a NaN@.
If you need the difference in units
of elapsed seconds, you can convert it with @code{time-convert} or
@code{float-time}. @xref{Time Conversion}.