The document discusses Daylight Saving Time (DST), which is the practice of advancing clocks during summer months so that evenings have more daylight. It causes issues for software like SAP around the time change in fall, as the clock repeats the same hour. The document describes three methods to address this "double hour" in SAP: 1) Shut down SAP for the full two hours, 2) Shut down for one hour, or 3) Slow the time in SAP so it sees the double hour as a single continuous hour. The zero downtime method of slowing time is the preferred approach.

1. Daylight Saving Time and Slowing Down The Time
What is Daylight Time Saving (or Is it Daylight Time Shifting)?
It is not until November 18, 1883 that US and Canada have adopted a standard time. Before
that time, the time keeping was a “local” matter. Some believe that Benjamin Franklin
conceived the idea of Daylight Saving Time. Whoever was behind this idea, the purpose of
Daylight Time Saving or changing the clock in Spring and Fall, seems to be to capture most
sunlit hours with given a typical day work schedule. Also an early goal of DST was to reduce
evening usage of incandescent lighting, formerly a primary use of electricity. But this is now
debatable as energy usage patterns have greatly changed since then.
Daylight Time Saving was first adopted by Germany and Austria in 1916 and soon after
adopted by many countries. US formally adopted the plan in 1918.
Daylight savings and clock adjustment:
There is also good history behind choosing which day the shifting of clock should happen. If
you are interested you can read more on one of the resources I have listed at the end of this
writing.
Currently adjustment of the clock is followed on the following days as shown below:
Adjustment of Clock for Daylight Savings in a year
When
Shift
clock? Daylight Saving Comment
2nd Sunday of March at 2am
Forward
by 1hr
Daylight Saving
Starts Spring forward
1st Sunday of November at 2am
Backward
by 1hr
Daylight Saving
Ends Fall Back
So, for example, in 2009:
On March 8th at 2am the clock was changed forward by 1hr to 3am.
On Nov 1st at 2am change the clock was changed backward by 1hr to 1am.

2. What is “Double hour”?
At the end of Daylight saving, on the 1st Sunday of November at 2am, you change your clock
backwards from 2am to 1am. This means the clock goes from 1am to 2am twice. Double hour
is this duration of the hour that your clock traverses twice from 1am to 2am.
Why should we care about Daylight Saving in SAP environment?
Almost all software that deals with local time will go bonkers when the computer clock goes
through the same points in time twice (as explained in “Double Hour” scenario above.). SAP
software is no exception to this either and is very sensitive to time. It expects time to continue
in a progressive and continuous manner. If not there can be problems in the planned batch
jobs. SAP Kernel being written in C/C++ uses C-runtime library functions and it can get
confused between standard time and local time.
How to deal with the “double hour”?
There are at least three ways the double hour problem can be dealt with. Which method you
adopt depends on your specific environment’s time zone settings and your tolerance for
system downtime.
1) Two hour downtime method: Avoid running SAP system during this double hour
completely.
2) One hour downtime method: Let SAP system run through the double hour only once,
but shutdown SAP to avoid go through the double hour twice.
3) Zero downtime method: Slow down the time in SAP System.
Now Let us examine each method.
Method-1) Two hour downtime
The first method is a simple and straight forward. You simply shutdown the SAP system for
the entire duration of double hour and bring it up only after the double hour passes. Thus the
SAP Kernel software does not have to deal with any possible mismatch of standard time and
local time. This means in practical terms for example in USA:
1) Shutdown SAP at 1am. (of course you should start shutting down a little earlier as
taking down the system takes a few minutes)
2) Keep SAP system down for the next two hours.
3) At 2am, your clock is changed from 2am to 1am for daylight saving time change.
4) Wait for another hour until 2am is reached.
5) Bring up SAP at 2am when the clock reaches this time again.
Disadvantages:
You need to keep the system down for 2 hours.

3. Method-2) One hour downtime
The second method is pretty much the same as the first one, but you should adopt this only in
cases where you know that all your SAP systems have identical local time settings. Then, you
can reduce the downtime close to 1 hour instead of 2 hours. In practical terms here is what
you can do:
1. Shutdown SAP a little before 2am before the clock is changed back to 1am.
2. Wait for one hour to pass and the clock comes back to 2am.
3. Bring up SAP.
This way your SAP system has gone through 1am to 2am only once. But you are keeping it
down for the clock’s 2nd pass of this one hour duration, thus avoiding ‘duplicate’ points in time.
Disadvantage:
You need to keep the system down for a little over 1 hour, though better than two hour
downtime as in method-1.
Method-3) Zero Downtime (Slow down the time)
Perception of slowing down time.
This is a very clever solution and my favorite one of all. In this case, you slow down the time
by half during the double hour so that it is only treated as one hour. In SAP it is easier than it
appears to be. You set the parameter zdate/DSTswitch_contloctime = on (By the way this can
be dynamically changed through RZ11), you can make your SAP system see a continuous time
with a deviation of 30 minutes. (Note that you should check the pre-requisites in the notes
mentioned at end of this article before you can do this).
In practical terms,
1. At 1am, that is one hour before the change of clock, your SAP system starts to see
time progress slowly at half speed.
2. For the next two hours SAP system will perceive time progress at half speed.
3. The double hour is hence seen as one continuous hour by SAP.
4. After two hours duration the clock time and SAP time will match and the system
continues normally then onwards with matched times.
For further reading, please refer to the resources at the end of this article.

4. Abbreviations:
DST – Daylight Saving Time
UTC – Universal Time Coordinated
Resources:
For a comprehensive history of daylight saving time, read
http://en.wikipedia.org/wiki/Daylight_saving_time
For some interesting incidents and anecdotes related to daylight saving, read
http://www.webexhibits.org/daylightsaving/k.html
For pre-requisites and official steps for any of the methods described above, read
OSS note: 438660 End of daylight-saving time coming soon, be ready!
OSS note: 102088 Reducing downtime when changing from summer to winter time
OSS note: 7417 Slowing time to adjust daylight saving clock without downtime.
OSS note: 950114 - Profile parameter zdate/DSTswitch_contloctime