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For a full 24 hours this week, Sprint will take over YouTube's homepage in an ad campaign featuring a "human clock," composed of dynamically-changing user-generated videos.

The effort is for Sprint's "Now Network." In what MediaPost dubbed a "first" for YouTube mastheads, will enable users to add themselves to the clock. Would-be participants are assigned a number — zero to nine — and use a web cam to shoot video and add themselves to the queue.

Videos will be screened by Sprint as they are submitted. Accepted ones will be strung together in a set of vignettes that represent the time, the way a digital clock would display it. "For example, at 12:09 the ad would display four people, each holding up one number to represent 1, 2, 0 and 9," MediaPost writes. The image changes as seconds, minutes and hours do.

The ad is also available as a widget at sprint.com/nownetwork, where the vignettes reflect local time — in addition to a panoply of other offerings intended to reflect "Now" — shopping days until Christmas, running Twitter tweets, top Google searches, and still others.

Digital ad manager Candice Wolken of Sprint dubbed the user-generated video effort "very appropriate" for both YouTube and the Now Network, because "it's all about 'Now'," she punned. She also said the campaign focuses on the meaning of the Now Network, rather than the fact that Sprint has it.

The creative was composed by Goodby Silverstein & Partners, which sought to make use of YouTube's "crosstalk ad unit," a high-impact masthead that debuted earlier this year. Larger than typical ad units, the masthead works with other ad units on the YouTube page to entertain users.

Product Manager Zal Bilimoria of YouTube says the rich media mastheads have gleaned 14% interaction rates, "compared to the industry average noted by DoubleClick for similar-size units of just under 5%," he beamed.

17 major advertisers across 12 countries have already purchased the mastheads, including Lionsgate, Electronic Arts, Apple and Universal.