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+;;; dawn.el --- Do things at dawn (and dusk) -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;; There is also circadian.el, but it doesn't quite work for me.
+;; This code comes mostly from https://gnu.xyz/auto_theme.html, but also
+;; somewhere else (which I've forgotten) and my own brain :)
+
+;;; Code:
+
+(require 'calendar)
+(require 'cl-lib)
+(require 'solar)
+
+(defvar dawn--dawn-timer nil
+  "Timer for dawn-command.")
+
+(defvar dawn--dusk-timer nil
+  "Timer for dusk-command.")
+
+(defvar dawn--reset-timer nil
+  "Timer to reset dawn at midnight.")
+
+(defun dawn-encode-time (f)
+  "Encode fractional time F."
+  (let ((hhmm (cl-floor f))
+        (date (cdddr (decode-time))))
+    (encode-time
+     (append (list 0
+                   (round (* 60 (cadr hhmm)))
+                   (car hhmm)
+                   )
+             date))))
+
+(defun dawn-midnight ()
+  "Return the time of the /next/ midnight."
+  (let ((date (cdddr (decode-time))))
+    (encode-time
+     (append (list 0 0 0 (1+ (car date))) (cdr date)))))
+
+(defun dawn-sunrise ()
+  "Return the time of today's sunrise."
+  (dawn-encode-time (caar (solar-sunrise-sunset (calendar-current-date)))))
+
+(defun dawn-sunset ()
+  "Return the time of today's sunset."
+  (dawn-encode-time (caadr (solar-sunrise-sunset (calendar-current-date)))))
+
+(defun dawn-schedule (dawn-command dusk-command)
+  "Run DAWN-COMMAND at sunrise, and DUSK-COMMAND at dusk.
+RESET is an argument for internal use."
+  (let ((dawn (dawn-sunrise))
+        (dusk (dawn-sunset)))
+    (cond
+     ((time-less-p nil dawn)
+      ;; If it isn't dawn yet, it's still dark.  Run DUSK-COMMAND and schedule
+      ;; DAWN-COMMAND and DUSK-COMMAND for later.
+      (funcall dusk-command)
+      (run-at-time dawn nil dawn-command)
+      (run-at-time dusk nil dusk-command))
+     ((time-less-p nil dusk)
+      ;; If it isn't dusk yet, it's still light.  Run DAWN-COMMAND and schedule
+      ;; DUSK-COMMAND.
+      (funcall dawn-command)
+      (run-at-time dusk nil dusk-command))
+     (t ;; Otherwise, it's past dusk, so run DUSK-COMMAND.
+      (funcall dusk-command)))
+    ;; Schedule a reset at midnight, to re-calculate dawn/dusk times.
+    ;(unless reset)
+    (run-at-time (dawn-midnight) nil
+                 #'dawn-schedule dawn-command dusk-command)))
+
+(provide 'dawn)
+;;; dawn.el ends here