Controlling cron

[Chuan Chee]

I have two problems:
(1) Suppose a cron is currently running.  I need to change the
    system time.  How do I make cron pick up the new time?
(2) Because I don't know the answer to the above question, my
    software currently calls a script that does something like:
        while there is a cron running
	     kill cron
	start a new cron
    Unfortunately there is a bug somewhere that makes it call this
    script twice in a row within 1 sec.  This is a very intermittent
    bug.  What happens is that the first script kills the cron and
    starts a new one.  However this doesn't appear in the process
    table ("ps -e") until it has loaded from disk.  The second script
    doesn't think there is one running so starts a (2nd) cron.

    Is there a way to have the system prevent 2 crons from running?
    Is there a way to make a cron "resident" so that it doesn't have
    to load from disk?