Limit the tasks (1 days worth) to envoke when the system clock
moves forward more than this.

Reference: /n/sources/patch/applied/cron-catchup
Date: Mon Dec  6 16:29:32 CET 2004

--- /sys/src/cmd/auth/cron.c	Mon Dec  6 16:29:32 2004
+++ /sys/src/cmd/auth/cron.c	Mon Dec  6 16:29:31 2004
@@ -106,6 +106,8 @@
 	for(;;){
 		readalljobs();
 		now = time(0) / 60;
+		if ((now-last) > (60*60*24))		/* don't go mad */
+			last = now-(60*60*24);
 		for(; last <= now; last += 2){
 			tm = *localtime(last*60);
 			t.min = 1 << tm.min/2;