[UPDATE: A few hours after this was posted, Chitika — without explanation — reset its counter to about 700,000. As the kids say: Fail.]

It happened Saturday night around 8:00 p.m. EDT. That’s when the big iPad counter that Chitika Labs has been running since April 5 rolled over like an odometer to 1,000,000.

If anywhere near accurate — a big if, given Chitika’s track record (see below) — that would mean it took Apple (AAPL) two weeks to sell its first million iPads.

By comparison, it took the company 74 days to sell a million first-generation iPhones, and three days each to sell a million units of the iPhone 3G and the iPhone 3Gs.

But can we trust Chitika’s counter?

The tracking device, which samples user data from Chitika‘s search-based network (some 2 billion ads served per month), has had its share of public pratfalls. When Steve Jobs announced on April 8 that Apple had sold 450,000 iPads, the number on Chitika’s counter was closer to 620,000.

“Well, it appears that our assumptions … have been way off,” wrote Chitika’s research director Dan Ruby that day, as he hastily reset the counter. It was reset again four days later as Ruby’s team switched from counting unique IP addresses attached to iPads to counting iPad cookies. The counter extrapolates based on the percentage of the Internet the network sees at any one time (roughly 15%) to calculate estimated unit sales.

“We’re pretty confident in our numbers,” said Ruby on Saturday. “I’m sure they’re off, as any estimate would be, but I’m confident we’re reasonably close.”

If Apple has indeed delivered more than 1,000,000 iPads, we’ll likely hear from its PR department soon enough — if not in a Monday morning press release, then on Tuesday when the company issues its second-quarter earnings report.

See also:

• Wednesday: 527,315 iPads and counting

[Follow Philip Elmer-DeWitt on Twitter @philiped]