time_shift parameter in trigger functions doesn't support time zones, so if you have a trigger expression with a time_shit parameter longer than a day, this can be a problem.

Here is an example:

sum(3600)-sum(3600,604800)

This trigger checks the absolute difference of the sum in the last hour and the sum of that same hour 7 days ago (3600*24*7=604800), so if the time zone changes during that week, the sum(3600,604800) will either use one hour earlier or later than the current hour.