Many countries and regions are operating Daylight Saving Time, which means for those of us in the Northern hemisphere, moving the clocks forward 1 hour in March/April and 1 hour back in October/November. Some Australasian regions also have daylight saving, but for these, the DST starts in October (clocks move forward) and finishes in March (clocks move back).
In Europe, the change is on the last Sunday in March, but a week later, i.e. the first Sunday in April for America and Canada. The change in October takes place on the last Sunday in October. From 2007, policy in the USA (with Canada following suit) will be changing to the 2nd Sunday in March and the 1st Sunday in November, under the Energy Policy Act of 2005. This change will be monitored by the US Department of Energy, and Congress retain the right to change it back if no significant energy savings are being made.
The change to and from DST takes place on the Sunday at 02:00 (am). Some atomic clocks such as the one at Boulder, Colorado encode a flag to indicate DST in their time transmissions, which receiving clocks can choose to process or ignore.
Daylight Saving and computers
The simplest appliances with clocks operate solely in local time, and need manually adjusting when DST changes. Modern video recorders have the logic and data of time zones, hence change automatically.
The Windows operating system has handled DST since Windows 95. This version (and 98) has a time dialog window opening with the message "Windows has updated your clock as a result of Daylight Saving Time", but this doesn't occur on NT based operating systems.
There is a potential problem with machines that are running time stamped transaction updates across the October/November change, in that the clock jumps backwards.
This can result in transactions ordered by time stamp getting out of sequence. The workaround in most cases is to bring the applications down over the period of DST change.
Unix operating systems work differently. The clock on a Unix box operates in Universal Coordinated Time (UTC), with a time zone offset. When DST changes, there is no change to the box's time, merely to the offset. Databases use UTC rather than local time, hence avoid the transaction problem.
The OpenVMS operating system has a different solution, with the DTSS component. Many legacy databases and transaction processing systems use system time, which is local time. DTSS implements UTC and time zones, running alongside system time. For daylight saving changes, DTSS operates a continuous drift between 00:00 and 04:00 on the day the DST change happens. This causes the system clock to run more quickly or more slowly during this interval, but timed events still happen in sequence. For a Spring change, the clock runs faster than normal, advancing 5 hours for 4 hours of elapsed time. For an Autumn change, the clock is only advanced 3 hours. NTP is based on DTSS.
Thank you Wntrmute, rootbeer277 and StrawberryFrog for suggestions that have been included in this writeup.