The best way to set Java/JVM options is to create a bin/setenv.sh or bin\setenv.bat file (depending on which OS you're using) and put the statement in that file. That way you don't have to modify the factory-supplied scripts.
You should, I believe, actually be able to simple make the setenv.sh contents look like this:
I don't believe that
Tomcat has any existing JAVA_OPTS values to worry about.
However, I should caution you that it is NOT a good idea to pass configuration information into webapps via the JVM environment.
From the looks of it, you are attempting to set a cron-style schedule (probably for Quartz Scheduler) and make it externally customizable. I'd recommend doing this with an Environment sub-element in the webapp's Context definition. It's what I always do.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.