This code implements some variants of CL:GET-UNIVERSAL-TIME with sub-second resolution for Clozure Common Lisp.

The times returned are based on whatever the function CCL:CURRENT-TIME-IN-NANOSECONDS returns, that is, whatever the OS returns. This means that currently leap seconds are ignored, the times reported can jump back and forth at random, etc. All the usual stuff which is also true for CL:GET-UNIVERSAL-TIME.

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• high-resolution-time-units-per-second

• (get-high-resolution-real-time)

  Like CL:INTERNAL-TIME-UNITS-PER-SECOND and CL:GET-INTERNAL-REAL-TIME, but with higher resolution.

• (current-nanoseconds-since-1900)

  What the name says.

• (get-universal-time-ii)

  Returns two values, the first value is the same as CL:GET-UNIVERSAL-TIME, the second value is a fraction of second expressed as an integer number of high resolution time units.

• (get-universal-time-ir)

  Returns two values, the first value is the same as CL:GET-UNIVERSAL-TIME, the second value is a fraction of second expressed as a rational.

• (get-universal-time-if)

  Returns two values, the first value is the same as CL:GET-UNIVERSAL-TIME, the second value is a fraction of second expressed as a long-float.

• (get-universal-time-r)

  Universal-time as a rational.

• (get-universal-time-f)

  Universal time as a long-float with precision of either the precision returned by CCL:CURRENT-TIME-IN-NANOSECONDS or the precision allowed by a LONG-FLOAT, whichever is smaller.

  On a system where LONG-FLOAT is IEEE-754 double and the timer has a resolution of 100 ns and the epoch is a few centuries back (e.g. CCL on Windows), in the the 21st century and beyond, the limiting factor is the precision of the float. That is, all of the digits of the float are significative and a little bit of additional resolution is lost (about 2 bits, in 2016):

  (- (log (/ (current-nanoseconds-since-1900) 100) 2l0)
     (float-digits 1l0))
  2.0332794619791557D0
  

  If maximum accuracy is desired use one of the other functions.

  (multiple-value-bind (s ns) (get-universal-time-ii)
    (let ((utf (get-universal-time-f)))
      (format t "~D.~9,'0D~%" s ns)       ; Maximum resolution.
      (format t "~F~%" utf)))             ; A few bits are lost.