The determination of document age is often of paramount importance when questioned documents are examined. In a physical setting, age determination may be based on static and/or dynamic principles. Static principles are based on the introduction dates of ink, paper and other writing materials. Since digital documents are based on artifacts that were often introduced on known dates, static methods to date documents have digital equivalents. Dynamic principles are based on the manner in which documents change over time. Such aging has no equivalent in the digital domain. This chapter introduces two principles that are used as a basis for determining document age. The principles are employed to determine a temporal creation window for a document, in a manner that makes it possible to convey the confidence within the window. When a creation window can be shown to predate or postdate a purported creation time or other critical event, it follows that the document cannot be authentic.