The AcuRite Large Wireless Atomic Clock with Indoor and Outdoor Temperature, Date, Day of the Week, and Automatic DST Changes (75440) is the perfect atomic clock for home use! It maintains time accuracy up to one second per million years! This 10.5-inch by 12.5-inch AcuRite atomic clock utilizes a built-in receiver to automatically pick up the National Institute of Standards and Technologies (NIST) radio signal from Fort Collins, Colorado. As a result, it displays and maintains exceptionally precise time and automatically updates each Spring and Fall for daylight saving time changes. The indoor clock has a built-in thermometer to measure and provide the indoor temperature, while an included outdoor wireless sensor thermometer takes the outdoor temperature and transmits that data to the indoor clock, which can display either degrees Fahrenheit or degrees Celsius. The large LCD display also provides today's date, month, and day of the week — making for the perfect clock with calendar for any room within your home or business. The indoor LCD clock takes three AA batteries (not included), and the outdoor thermometer takes two AA batteries (not included), which results in a completely wireless thermometer clock. This AcuRite atomic clock is a perfect gift for Christmas, Father's Day, or birthdays and comes with a one-year limited warranty.