Hollywood celebrities have been known to deduct a few numbers when declaring their ages. Turns out Silicon Valley startups do the same thing.

In a business where everyone is searching for the next big disruptive concept, old age is rarely considered an asset. As such, some companies make their stated dates of founding subject to change.

Chris O’Neill, chief executive of Evernote Corp., uses June 24, 2008—the day the company’s note-taking iPhone app launched—as the company’s cornerstone date. But Evernote’s roots go back further: Its founder wrote the first lines of code in 2000 and then formed the company in 2002. It released its first product—a Windows PC application that it now describes as a prototype of its iPhone app—in 2004.

“There are a lot of important dates in a company’s history,” Mr. O’Neill said. “Our point of view is, ‘Let’s pick the most important date.’ ”

Evernote, which has had three chief executives, has at various times pointed to 2002, 2005 or 2007 as its founding year.