Groupon gets you deals on your lunch.

Google may have launched its first daily deal offering in Portland, Ore., yesterday, but Groupon is fighting back one city at a time, with a spin on the model it popularized.

Groupon Now, which rolled out in San Francisco and New York yesterday and launched in Chicago two weeks ago, follows the formula of Groupon's original online daily deal but shrinks the timeline. Instead of having 24 hours to purchase a perk and months to use it, you'll now have hours to both buy the deal and cash it in.

The premise is simple. After entering your location on the Groupon Now site or opening the Groupon app on your iPhone or Android phone, you choose from the categories in which you're interested, like restaurants, shopping, flowers, and spa treatments. Then, you scan through the options that pop up on a Google map of your area. You'll see either a countdown of the hours you have left to use the deal, or a range of available hours, like 9 a.m. to 10 p.m., for that day only.

The service also offers businesses, such as local restaurants and vendors, a chance to cycle through inventory on slower days by offering cut-rate deals that have firm starting and ending times. By choosing the times and dates that customers can cash in their coupons, businesses can use the new structure to prepare for the influx in a way they couldn't with the original daily deal setup.

The fact that the instant, limited-time offer follows the model of an instant auction site more than the deal-a-day format Groupon itself popularized is also a sign that Groupon is still the pioneer in the broader online coupon industry now choked with competitors from Living Social to Yelp to Google.

Groupon is keeping the schedule of its remaining Now deployment hush-hush for the time being, but if you're a registered Groupon user, you should receive an e-mail alert when the instant deals become available in your city.

Update, 11:20am PT: adds details about Groupon Now, and a corrected San Francisco launch date.