Why does Google celebrate its birthday on Sept 27 despite being founded on Sept 4?

Google celebrates its birthday on September 27, but it was incorporated on September 4, 1998. Earlier, Google marked the occasion on different dates, including September 7, 8 and 26, before settling on September 27 in the mid-2000s. Google has not explained the choice, though the date has been linked to its 2005 announcement of a much larger web search index.