I think you just need the "function"
date on its own. That gives you the time now as the number of seconds since 1970, the reference you want is http://doc.tiki.org/Calculations#date not really the PHP
So probably something like: (untested)
(date (str H:i) (sub futureDate date))
str H:i part gives you the format, and those you can look up on https://www.php.net/manual/en/datetime.format.php