>;; time arithmetic
>
>(add-seconds! time real) => time adds number of seconds to date,
> side-effecting it
>(add-seconds time real) => time adds number of seconds to date,
>creating new date.

There seems to be a missing operator here:

(time-difference time1 time2) => returns number of seconds (real)
elapsed from time2 to time1 (positive if time1 is later than time2).

-- Prfnoff
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