Can someone explain when this isn't true? Is he referring to leap seconds, or local timezone DST changes? Or something more interesting I'm failing to think of?
DST changes are a big issue. A simple example from real life - there is a system that takes a measurement every hour, 24/7. Make a report that prints a table of the historical measurements for each day. Does your report show correctly that some days have 24 rows, some have 25 rows and some 23?
Can someone explain when this isn't true? Is he referring to leap seconds, or local timezone DST changes? Or something more interesting I'm failing to think of?