Automatic DST changes in VCRs

Markus Kuhn mskuhn at
Sat Apr 20 09:13:00 UTC 1996

Chris Carrier wrote:

> About time changes being automatically built into software, VCRs,
> etc... I regard this as a very BAD idea, because DST rules
> change.

No, I do not agree.

Building the rules into VCRs is an excellent idea, if there is a way
to do over-the-air upgrades! In Europe, practically all VCRs have
already built in a small modem that receives teletext, current time,
information that allows to do precise programmed recordings even if
the broadcast time has been changed from the programmed one (VPS =
Video Programming System), data for sex/violence cencoring mechanisms
for parents, etc.

These radio data services (available in Germany since around 1980)
could easily also broadcast every few hours the TZ rules for all
countries in which reception of the service is possible (e.g. in the
POSIX TZ format or in a data structure similar to the one used in the
famous Olson package). This way, the VCR (as well as the TV, the
cellular phone, the sat receiver, etc.) could upgrade their time zone
algorithms automatically in NVRAM if the rules change.

The user would only have to select during installation, in which
country (or in large countries like the US: in which time zone region)
she is living, and from then on, the time would never have to be
changed again, no matter how creative you politicians are.

The above mentioned radio data services broadcast already time signals
and many VCRs and TV sets can adjust their clocks to these time
signals automagically. The problem is only: National terrestrial
broadcasters send local time, which can cause problems if you receive
the program via satellite on another continent. Therefore, TV sets can
be configured so that only the time signal of one local channel is the
"trusted" clock signal that adjust the local clock.

A better approach is the system information broadcasted with the new
EBU Digital Video Broadcast (DVB) standard (an extension of the MPEG
II digital TV standard). The DVB standard adds very comprehensive
electronic program guide information to the digital TV signal. All
dates and times in this electronic program guide as well as the
current time are broadcasted in UTC. [The data format is a 40-bit
field with the lower 16-bit of the MJD (days since 1900-01-01 00:00Z)
and 24-bits for hhmmss coded in 4-bit BCD.]

It is the duty of the receiver set-top box to convert all broadcasted
UTC times into local time (LT). One DVB receiver I have seen so far
has a configuration menu option that allows to adjust the LT-UTC
difference in 30 min steps.

In my opinion, the better approach would be to offer a country/region
menue such that the user only has to select the time zone rule once
and the receiver can automatically upgarde its TZ rules from
periodically broadcasted time zone rules data.

The fact that this nice TZ update solution has been nowhere
implemented so far indicates only, that many software and standards
developpers (readers of this mailing list excluded of course ;-) are
still ignorant of time zone issues. It would be nice for example if
the next revision of the DVB-SI standard would be extended by a time
zone rules broadcast mechanism.


  - Digital broadcasting systems for television, sound and data
    services; Specification for Service Information (SI)
    in Digital Video Broadcasting (DVB) systems, ETS 300 468,
    European Telecomunication Standards Institute, October 1995.


Markus Kuhn, Computer Science student -- University of Erlangen,
Internet Mail: <mskuhn at> - Germany
WWW Home: <>

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