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These functions perform calendrical computations using time values
(see Time of Day). A value of `nil`

for any of their
time-value arguments stands for the current system time, and a single
integer number stands for the number of seconds since the epoch.

— Function: **time-subtract**` t1 t2`

This returns the time difference

t1−t2between two time values, as a time value. If you need the difference in units of elapsed seconds, use`float-time`

(see float-time) to convert the result into seconds.

— Function: **time-add**` t1 t2`

This returns the sum of two time values, as a time value. One argument should represent a time difference rather than a point in time, either as a list or as a single number of elapsed seconds. Here is how to add a number of seconds to a time value:

(time-addtimeseconds)

— Function: **time-to-days**` time-value`

This function returns the number of days between the beginning of year 1 and

time-value.