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Maxim > Design Support > Technical Documents > Application Notes > Real-Time Clocks > APP 3778
Keywords: DST,Daylight Saving Time,Daylight Savings Time
APPLICATION NOTE 3778
Problems and Solutions for Adjusting to Changes
in Daylight Saving Time
Mar 23, 2006
Abstract: Recent U.S. legislation increased the daylight saving time (DST) period by four weeks. This
change will affect systems that adjust for DST by using real-time clocks (RTCs) with an integrated DST
adjustment. This note discusses issues related to the DST change, shows how to test and adjust the
RTC for DST, and recommends how to manage DST in future designs.
Introduction and Background on DST Implementation
Since 1986 daylight saving time, in the U.S., begins on the first Sunday in April and ends on the last
Sunday in October. Canada and Mexico also follow this schedule¹. Beginning in 2007, however, daylight
saving time in the U.S. will begin on the second Sunday in March and end on the first Sunday in
November².
Historically the DS1287 RTC, used on the IBM AT Personal Computer (PC), integrated the U.S. DST
function. If the DST function was enabled, the operation system's (OS) clock used the time and date
information from the RTC at boot time to reflect the proper time. For legacy reasons, all other Dallas
Semiconductor multiplexed-bus interface RTCs also support the DST function. (See Table 1.)
Table 1. Multiplexed-Bus RTCs with Integrated DST Operation
DS12885/87 DS12R885/CR887/R887 DS14285/87
DS1685/87 DS1688/91 DS1689/93
DS17285/87 DS17485/87 DS17885/87
In modern operating systems the OS, not the RTC, handles the DST function. Since DST is
implemented differently worldwide and can change often, using the OS allows relatively easy updates to
the DST function and DST support. Embedded systems, in contrast, often have limited software,
firmware, or memory resources, are limited to sales in North America, and may still implement DST by
using the RTC's integrated DST function.
The DST function is important in some applications where there is no OS DST adjustment. Traffic signal
controllers, for example, adjust the signal timing during morning and evening rush hours, which shift with
DST. Other applications, such as FAX machines, may depend upon the user to input the correct time
when a DST shift occurs, and therefore have little need for automatic DST adjustment.
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