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Parsing formatted time string (inverse of format-time-string) asks for a complete inverse of format-time-string with the ability to specify the source format, but I really just need to get from a UNIX time / epoch time to a real time value (like that which (current-time) returns.

In essence, I want the following:

(let* ((emacs-time (current-time))
       (unix-time  (string-to-number (format-time-string "%s" emacs-time))))
  (equal emacs-time (mystery-func unix-time)))
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In keeping with tradition, I was able to find the answer in Emacs itself while asking this question.

It turns out the standard time functions work with different kinds of values:

Function arguments, e.g., the TIME argument to current-time-string, accept a more-general "time value" format, which can be a list of integers as above, or a single number for seconds since the epoch, or nil for the current time. You can convert a time value into a human-readable string using current-time-string and format-time-string, into a list of integers using seconds-to-time, and into other forms using decode-time and float-time. These functions are described in the following sections.

So, seconds-to-time will do what's needed. Note that since (current-time) returns microseconds as well – something not present in most UNIX timestamps – the assertion in the question won't hold as-written. If you strip microseconds from (current-time), though, they'll be the same :-)

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