How to periodically run a task within emacs?

Is there a way to periodically run an elisp function in a long-running emacs, similar to cron, but within the emacs process?

For example I want to "automatically run (recentf-save-list) every half hour" because it otherwise only runs on exit, which sucks when emacs occasionally crashes.开发者_开发技巧 (There are other examples as well so looking for a general solution rather than one in particular for recentf).

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Check out run-with-timer.

(run-with-timer 0 (* 30 60) 'recentf-save-list)

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You might also find midnight mode useful. One can arbitrarily define 'midnight' and then add hooks as desired.