seperating user timezones from system timezones

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How to I allow users access to their own timezone without affecting the
system processes?

All of my systems regardless of location have always been set to UTC so
logs and cron are in sync across timezones.

Recently I have had need to allow users to set their own timezone in the
.cshrc using:
sentenv TZ America/Detroit
or
sentenv TZ America/Denver
or whatever applies.

But what I am finding out is that as long as the user is logged in it
sets the environment for the entire system affecting log timestamps as
well as cron events.

Any help is appreciated.
Jason Cribbins