[tz] How to compute the moment of the next DST change

[Simon Perreault]

Dear time experts,

I want to write a cron job or similar that activates one week prior to 
daylight saving time changes. (Or, more generically, one week prior to 
any time jumps, for any reason, DST or else.)

Any pointers?

I have looked at the usual places: POSIX time functions, various 
date/time modules on CPAN, etc. Nothing seems to do that computation. Do 
I need to parse the tz database myself?

Thanks,
Simon