+std::string const formatted_time(time_t t, std::string const & fmt);
+
+/**
+ * Inverse of ctime().
+ * Since ctime() outputs the local time, the caller needs to ensure that the
+ * time zone and daylight saving time are the same as when \p t was created
+ * by ctime().
+ */
+time_t from_ctime(std::string t);