Why is the date_diff() function off an hour in my report?

Environment:

Analytics, Sites which utilize Daylight Savings Time (DST)
Oracle B2C Service, All versions

Issue:

A report is off an hour when using the date_diff() function to calculate a time interval from midnight to a time later in the day.

Resolution:

Daylight Savings Time (DST) is the practice of setting the clock forward by one hour during the warmer part of the year so that evenings have more daylight and mornings have less. DST commonly starts at 2:00 am local time and DST will move the clocks forward one hour. Using a date_diff() function in a report that expands the actual time change and is calculating the number of seconds between two times can be short an hour during DST and long an hour when DST ends.

The date_diff() function will calculate the actual physical seconds taking place between the two times.