A calendar that will outlast the one in your cell phone, this polished brass piece makes a terrific gift. It is an early, simple pocket calculator that dates back some 500 years. To calculate the given date within a 100-year time span between 2019 and 2118, simply rotate the dial until the selected year lines up with the selected month. From there, the date and day of the week can be located below.

One of the oldest scientific agencies in the U.S., the United States Naval Observatory in northwest Washington, D.C., is one of few pre-1900 astronomical observatories located in an urban area. The USNO operates the Master Clock, which is the underlying product for all of our precise time and time interval products. The timing reference produced by this timing ensemble is called UTC (USNO). This timing reference is mandated to be the precise time reference for all of the Department of Defense.