IronPort Systems 4108GL - Manual (Page 469)

C-3
Daylight Savings Time on HP Procurve Switches
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Before configuring a "User defined" Daylight Time Rule, it is important to
understand how the switch treats the entries. The switch knows which dates
are Sundays, and uses an algorithm to determine on which date to change the
system clock, given the configured "Beginning day" and "Ending day":
■ If the configured day is a Sunday, the time changes at 2am on that day.
■ If the configured day is not a Sunday, the time changes at 2am on the first
Sunday after the configured day.
This is true for both the "Beginning day" and the "Ending day".
With that algorithm, one should use the value "1" to represent "first Sunday of
the month", and a value equal to "number of days in the month minus 6" to
represent "last Sunday of the month". This allows a single configuration for
every year, no matter what date is the appropriate Sunday to change the clock.