OCXO Digital Wall Clocks
Plug these digital wall clocks in and you won't need to adjust the time for the next 20 years. No master clock, visibility to GPS, or access to CDMA signals is required. You can bury one of these clocks in a bunker and it will keep rock-solid time accurate to 1 second in 20 years.
What is OCXO?
OCXO stands for Oven-Controlled Crystal Oscillator, the heart of these clocks. In a nutshell, quartz clocks keep time by counting the oscillations in a quartz crystal. Temperature variations can cause the frequency of the oscillations to vary, which affects the accuracy of the clock. The oven in an OCXO keeps the quartz crystal at a very stable temperature so that the frequency of the quartz oscillations is equally stable. The net result is very accurate timekeeping.
Why Do These Clocks Plug In?
Unfortunately, a battery pack of any reasonable size would be drained too quickly. The crystal oven in an OCXO maintains the temperature of the quartz wafer at a level well above what will be encountered in everyday use, typically around 170 °F (~75 °C). Maintaining that temperature takes power.
Why Not Maintain a Lower Temperature in the OCXO?
If the OCXO temperature is maintained at a lower level, say 100 °F (~38 °C), then a clock in environments where the temperature exceeds 100 °F will start to lose precision due to temperature-driven variations in the quartz oscillation. That would be bad. So, sure, the crystal could be kept at a lower temperature, but only if you were absolutely sure the ambient temperature of the operating environment would never exceed the temperature maintaind by the crystal oven. You know, like in the arctic, with no heaters.
20 Years? Really?
Yes, really. Thanks to the OCXO, these clocks are accurate to one second in 20 years. The only things that can affect this are moving the clock to a new time zone and when The Powers That Be (The International Earth Rotation and Reference Systems Service, if you really want to know) decide to add or remove a leap second, which occurred 27 times in the 44 years between 1972 and 2016. If either of those happen, then the clock may be adjusted with factory-supplied software.
These are super-precise digital wall clocks that have to be plugged in due to power requirements beyond what a battery pack can effectively supply. After plugging in one of these clocks, the only time you should have to adjust it is if a leap second is added or you move the clock to a new time zone. Other than that, you should be good to go for 20 years.