Compare the serial number with known serial numbers and years of production.
It gets the energy it needs to run from temperature and atmospheric pressure changes in the environment, and can run for years without human intervention.
Clocks made in the 1990s have a serial number in the 650,000 to 704,000 range.
Though there is no known published reference books that date or 'grade' Atmos Clocks based on age or condition, the Chart below may be helpful in determining the age of your Atmos Clock.
A good way of determining the approximate value of your particular clock is to regularly visit on-line Auction Sites; look for clocks at auctions similar to yours, comparing your clock to identical ones with the final "winning bid amounts".
Prices at auction sites are often reflective of the current market value of most any item.