The manual says it has a lithium battery good for 5 years. Probably conservative.
There is also a big section on how to set it if the battery dies.
Our clock radio in our bedroom is an Emerson, although not the same model as in Lips_Obsession’s link. And that’s exactly how it works. The time is preset at the factory, and it has a battery backup to store it until you plug it in. There is a switch on the back to set it to one of four time zones to display the correct time. It also has a DST switch, which I have turned off because it looks like it was programmed around the old DST start & end dates. (I fake DST by just shifting the time zone ahead or back one.)
The WWV signal does contain a DST indicator in bits 57 and 58. Devices that get the time from WWV can and do use these bits to adjust for DST.
NIST Time and Frequency Services (NIST Special Publication 432) – pdf
And, FYI, the signal also contains leap second and leap year bits.
Yes, I had a Sony clock that worked similarly. Note that these clocks don’t appear to adjust for leap seconds.
That’s what my Sony clock did, so it’s not all that accurate when you turn it on.
Some TV channels used to broadcast a time signal (in NY, channel 13, the PBS channel, did) and I have a VCR that used to set itself. Now that those analog stations are all turned off, I think, I doubt that would still work.
I doubt it’s using GPS, wi-fi, or anything like that.
One radio time system that I’m surprised to have not seen built in to anything (except perhaps some cars) is RDS/RBDS over the FM band.
This is a digital signal overlaid on analog FM radio and contains information like the name of the current song. It also contains the current time. A self-setting clock wouldn’t need to be programmed to the local stations; it could just scan through the FM band at the normal channel spacing, listening for a few tens of seconds to see if it got a correct time message. The protocol is simple and shouldn’t cost more than a dollar to build into a device, and would work anywhere that FM can penetrate (i.e., much better than GPS or NIST). Not all stations broadcast RBDS but you just need one.
Good to know but that still leaves TZ setting which may or may not be smart enough to handle the places that don’t observe DST. And of course there’s the problem of states like Montana that are considering dropping DST.
To elaborate: The grid frequency is slightly low during peak demand times. The power utility counts cycles, and makes up the missing ones at night. They hit the correct number of cycles per day, and if they miss for some reason they fix it the next day. With computers, there is seldom a miss. Of course this is not true for outages.
In the 1990’s I worked on a project for radio station automation. As part of that I was at a WKRP-like station in York, PA for a week here, and another week there. They carried some syndicated programs via a satellite dish on the roof. These started on-the-dot time wise, but the station had only a simple analog AC clock for the DJs to time the intros…which would drift during the day.
The low tech solution was a sticky note with an arrow on it showing the position of the second hand at the top-of-the-minute. The DJ’s would track the satellite time hacks prior to the start of a sat. program, and move the sticky note around the clock as required. It would move within about a 15 second window over the day.