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I noticed there is no standard time function for scheme.
Let me propose the following:
(days-after-J2000.0) => current time in units of days
as a floating point number. The time origin is noon,
January 1, 2000. This date is called J2000.0 by astronomers,
and represents a good time origin for those who are interested
in computing the position of the sun and other stars. For
example, a formula that gives the approximate location of the
sun in these units is in "The Astronomical Almanac for the Year
1984", US Naval Observatory and Royal Greenwich Observatory,
US Government Printing Office, Washington DC, 1984.