Context: Changes to DST rules cause problems in existing computer installations. For example, the 2007 change to DST rules in North America required many computer systems to be upgraded, with the greatest impact on email and calendaring programs; the upgrades consumed a significant effort by corporate information technologists.

Question: What can cause issues with installed computer systems?
Answer: Changes to DST rules

Question: What year did DST rules change in North America?
Answer: 2007

Question: What did many existing computer systems that relied on time zones need because of the DST rule changes?
Answer: upgrades

Question: In addition to calendaring programs, what kind of programs were most impacted by the changes?
Answer: email

Question: Which professionals bore the brunt of the effort to upgrade systems affected by time changes?
Answer: corporate information technologists